Gating mean for metal matrix composite manufacture

ABSTRACT

The present invention relates to the use of a gating means in combination with a spontaneous infiltration process to produce a metal matrix composite body. Particularly, a permeable mass of filler material or a preform is spontaneously infiltrated by molten matrix metal to form a metal matrix composite body. A gating means is provided which controls or limits the areal contact between molten matrix metal and the filler material or preform. The use of a gating means provides for control of the amount of matrix metal which can contact the preform or filler material, which may result in less machining of a formed metal matrix composite body compared with a similar metal matrix composite body made without a gating mean. Moreover, the use of a gating means ameliorates the tendency of a formed metal matrix composite body to warp due to the contact between the formed composite body and matrix metal carcass.

This is a continuation of application Ser. No. 08/261,694 filed on May31, 1994 which issued on Sep. 10, 1996 as U.S. Pat. No. 5,553,657, whichis a continuation of U.S. application Ser. No. 07/961,680, filed on Jan.8, 1993, and now abandoned, which was a continuation-in-part of U.S.application Ser. No. 07/521,196, filed May 9, 1990, which issued on Jun.9, 1992 as U.S. Pat. No. 5,119,564.

TECHNICAL FIELD

The present invention relates to the use of a gating means incombination with various metal infiltration processes which may beutilized to produce a metal matrix composite body. Particularly, apermeable mass of filler material or a preform is infiltrated by moltenmatrix metal (e.g., spontaneously, by pressure infiltration, by vacuuminfiltration, etc.) to form a metal matrix composite body. For example,in a preferred method for forming metal matrix composite bodies by aspontaneous infiltration process, an infiltration enhancer and/or aninfiltration enhancer precursor and/or an infiltrating atmosphere arealso in communication with the filler material or preform, at least atsome point during the process, to permit the molten matrix metal tospontaneously infiltrate the filler material or preform. Moreover, agating means is provided which controls or limits the areal contactbetween molten matrix metal and the filler material or preform. The useof a gating means provides for control of the amount of matrix metalwhich can contact the preform or filler material. Such limited orcontrolled areal contact may result in less required machining of aformed metal matrix composite body to achieve a net or near-net shapebody as compared to a similar metal matrix composite body made without agating means. Moreover, the use of a gating means ameliorates thetendency of a formed metal matrix composite body to warp due to thecontact between the formed composite body and matrix metal carcass. Suchwarping may be the most prevalent in metal matrix composite bodies whichhave a high surface area relative to cross-sectional thickness.

BACKGROUND ART

Composite products comprising a metal matrix and a strengthening orreinforcing phase such as ceramic particulates, whiskers, fibers or thelike, show great promise for a variety of applications because theycombine some of the stiffness and wear resistance of the reinforcingphase with the ductility and toughness of the metal matrix. Generally, ametal matrix composite will show an improvement in such properties asstrength, stiffness, contact wear resistance, and elevated temperaturestrength retention relative to the matrix metal in monolithic form, butthe degree to which any given property may be improved depends largelyon the specific constituents, their volume or weight fraction, and howthey are processed in forming the composite. In some instances, thecomposite also may be lighter in weight than the matrix metal per se.Aluminum matrix composites reinforced with ceramics such as siliconcarbide in particulate, platelet, or whisker form, for example, are ofinterest because of their higher stiffness, wear resistance and hightemperature strength relative to aluminum.

Various metallurgical processes have been described for the fabricationof aluminum matrix composites, including methods based on powdermetallurgy techniques and liquid-metal infiltration techniques whichmake use of pressure casting, vacuum casting, stirring, and wettingagents. With powder metallurgy techniques, the metal in the form of apowder and the reinforcing material in the form of a powder, whiskers,chopped fibers, etc., are admixed and then either cold-pressed andsintered, or hot-pressed. The maximum ceramic volume fraction in siliconcarbide reinforced aluminum matrix composites produced by this methodhas been reported to be about 25 volume percent in the case of whiskers,and about 40 volume percent in the case of particulates.

The production of metal matrix composites by powder metallurgytechniques utilizing conventional processes imposes certain limitationswith respect to the characteristics of the products attainable. Thevolume fraction of the ceramic phase in the composite is limitedtypically, in the case of particulates, to about 40 percent. Also, thepressing operation poses a limit on the practical size attainable. Onlyrelatively simple product shapes are possible without subsequentprocessing (e.g., forming or machining) or without resorting to complexpresses. Also, nonuniform shrinkage during sintering can occur, as wellas nonuniformity of microstructure due to segregation in the compactsand grain growth.

U.S. Pat. No. 3,970,136, granted Jul. 20, 1976, to J. C. Cannell et al.,describes a process for forming a metal matrix composite incorporating afibrous reinforcement, e.g. silicon carbide or alumina whiskers, havinga predetermined pattern of fiber orientation. The composite is made byplacing parallel mats or felts of coplanar fibers in a mold with areservoir of molten matrix metal, e.g., aluminum, between at least someof the mats, and applying pressure to force molten metal to penetratethe mats and surround the oriented fibers. Molten metal may be pouredonto the stack of mats while being forced under pressure to flow betweenthe mats. Loadings of up to about 50% by volume of reinforcing fibers inthe composite have been reported.

The above-described infiltration process, in view of its dependence onoutside pressure to force the molten matrix metal through the stack offibrous mats, is subject to the vagaries of pressure-induced flowprocesses, i.e., possible non-uniformity of matrix formation, porosity,etc. Non-uniformity of properties is possible even though molten metalmay be introduced at a multiplicity of sites within the fibrous array.Consequently, complicated mat/reservoir arrays and flow pathways need tobe provided to achieve adequate and uniform penetration of the stack offiber mats. Also, the aforesaid pressure-infiltration method allows foronly a relatively low reinforcement to matrix volume fraction to beachieved because of the difficulty inherent in infiltrating a large matvolume. Still further, molds are required to contain the molten metalunder pressure, which adds to the expense of the process. Finally, theaforesaid process, limited to infiltrating aligned particles or fibers,is not directed to formation of aluminum metal matrix compositesreinforced with materials in the form of randomly oriented particles,whiskers or fibers.

In the fabrication of aluminum matrix-alumina filled composites,aluminum does not readily wet alumina, thereby making it difficult toform a coherent product. Various solutions to this problem have beensuggested. One such approach is to coat the alumina with a metal (e.g.,nickel or tungsten), which is then hot-pressed along with the aluminum.In another technique, the aluminum is alloyed with lithium, and thealumina may be coated with silica. However, these composites exhibitvariations in properties, or the coatings can degrade the filler, or thematrix contains lithium which can affect the matrix properties.

U.S. Pat. No. 4,232,091 to R. W. Grimshaw et al., overcomes certaindifficulties in the art which are encountered in the production ofaluminum matrix-alumina composites. This patent describes applyingpressures of 75-375 kg/cm² to force molten aluminum (or molten aluminumalloy) into a fibrous or whisker mat of alumina which has been preheatedto 700° to 1050° C. The maximum volume ratio of alumina to metal in theresulting solid casting was 0.25/1. Because of its dependency on outsideforce to accomplish infiltration, this process is subject to many of thesame deficiencies as that of Cannell et al.

European Patent Application Publication No. 115,742 describes makingaluminum-alumina composites, especially useful as electrolytic cellcomponents, by filling the voids of a preformed alumina matrix withmolten aluminum. The application emphasizes the non-wettability ofalumina by aluminum, and therefore various techniques are employed towet the alumina throughout the preform. For example, the alumina iscoated with a wetting agent of a diboride of titanium, zirconium,hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium,titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inertatmospheres, such as argon, are employed to facilitate wetting. Thisreference also shows applying pressure to cause molten aluminum topenetrate an uncoated matrix. In this aspect, infiltration isaccomplished by evacuating the pores and then applying pressure to themolten aluminum in an inert atmosphere, e.g., argon. Alternatively, thepreform can be infiltrated by vapor-phase aluminum deposition to wet thesurface prior to filling the voids by infiltration with molten aluminum.To assure retention of the aluminum in the pores of the preform, heattreatment, e.g., at 1400° to 1800° C., in either a vacuum or in argon isrequired. Otherwise, either exposure of the pressure infiltratedmaterial to gas or removal of the infiltration pressure will cause lossof aluminum from the body.

The use of wetting agents to effect infiltration of an alumina componentin an electrolytic cell with molten metal is also shown in EuropeanPatent Application Publication No. 94353. This publication describesproduction of aluminum by electrowinning with a cell having a cathodiccurrent feeder as a cell liner or substrate. In order to protect thissubstrate from molten cryolite, a thin coating of a mixture of a wettingagent and solubility suppressor is applied to the alumina substrateprior to start-up of the cell or while immersed in the molten aluminumproduced by the electrolytic process. Wetting agents disclosed aretitanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium,niobium, or calcium, and titanium is stated as the preferred agent.Compounds of boron, carbon and nitrogen are described as being useful insuppressing the solubility of the wetting agents in molten aluminum. Thereference, however, does not suggest the production of metal matrixcomposites, nor does it suggest the formation of such a composite in,for example, a nitrogen atmosphere.

In addition to application of pressure and wetting agents, it has beendisclosed that an applied vacuum will aid the penetration of moltenaluminum into a porous ceramic compact. For example, U.S. Pat. No.3,718,441, granted Feb. 27, 1973, to R. L. Landingham, reportsinfiltration of a ceramic compact (e.g., boron carbide, alumina andberyllia) with either molten aluminum, beryllium, magnesium, titanium,vanadium, nickel or chromium under a vacuum of less than 10⁻⁶ torr. Avacuum of 10⁻² to 10⁻⁶ torr resulted in poor wetting of the ceramic bythe molten metal to the extent that the metal did not flow freely intothe ceramic void spaces. However, wetting was said to have improved whenthe vacuum was reduced to less than 10⁻⁶ torr.

U.S. Pat. No. 3,864,154, granted Feb. 4, 1975, to G. E. Gazza et al.,also shows the use of vacuum to achieve infiltration. This patentdescribes loading a cold-pressed compact of AlB₁₂ powder onto a bed ofcold-pressed aluminum powder. Additional aluminum was then positioned ontop of the AlB₁₂ powder compact. The crucible, loaded with the AlB₁₂compact "sandwiched" between the layers of aluminum powder, was placedin a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵torr to permit outgassing. The temperature was subsequently raised to1100° C. and maintained for a period of 3 hours. At these conditions,the molten aluminum penetrated the porous AlB₁₂ compact.

U.S. Pat. No. 3,364,976, granted Jan. 23, 1968, to John N. Reding etal., discloses the concept of creating a self-generated vacuum in a bodyto enhance penetration of a molten metal into the body. Specifically, itis disclosed that a body, e.g., a graphite mold, a steel mold, or aporous refractory material, is entirely submerged in a molten metal. Inthe case of a mold, the mold cavity, which is filled with a gas reactivewith the metal, communicates with the externally located molten metalthrough at least one orifice in the mold. When the mold is immersed intothe melt, filling of the cavity occurs as the self-generated vacuum isproduced from the reaction between the gas in the cavity and the moltenmetal. Particularly, the vacuum is a result of the formation of a solidoxidized form of the metal. Thus, Reding et al. disclose that it isessential to induce a reaction between gas in the cavity and the moltenmetal. However, utilizing a mold to create a vacuum may be undesirablebecause of the inherent limitations associated with use of a mold. Moldsmust first be machined into a particular shape; then finished, machinedto produce an acceptable casting surface on the mold; then assembledprior to their use; then disassembled after their use to remove the castpiece therefrom; and thereafter reclaim the mold, which most likelywould include refinishing surfaces of the mold or discarding the mold ifit is no longer acceptable for use. Machining of a mold into a complexshape can be very costly and time-consuming. Moreover, removal of aformed piece from a complex-shaped mold can also be difficult (i.e.,cast pieces having a complex shape could be broken when removed from themold). Still further, while there is a suggestion that a porousrefractory material can be immersed directly in a molten metal withoutthe need for a mold, the refractory material would have to be anintegral piece because there is no provision for infiltrating a loose orseparated porous material absent the use of a container mold (i.e., itis generally believed that the particulate material would typicallydisassociate or float apart when placed in a molten metal). Stillfurther, if it was desired to infiltrate a particulate material orloosely formed preform precautions should be taken so that theinfiltrating metal does not displace at least portions of theparticulate or preform resulting in a non-homogeneous microstructure.

Accordingly, there has been a long felt need for a simple and reliableprocess to produce shaped metal matrix composites which does not relyupon the use of applied pressure or vacuum (whether externally appliedor internally created), or damaging wetting agents to create a metalmatrix embedding another material such as a ceramic material. Moreover,there has been a long felt need to minimize the amount of finalmachining operations needed to produce a metal matrix composite body.The present invention satisfies these needs by providing a mechanism forinfiltrating a filler material or preform with molten matrix metal(e.g., aluminum) with the use of a gating means which provides forcontrol of the amount of matrix metal which can contact the fillermaterial or preform, thereby resulting in less required machining of theformed metal matrix composite body to achieve a net or near-net shapebody, as compared to a metal matrix composite body formed without theuse of gating means.

DESCRIPTION OF COMMONLY OWNED U.S. PATENTS AND PATENT APPLICATIONS

The subject matter of this application is related to that of severalco-owned Patents and several other copending and co-owned patentapplications. Particularly, the patents and other copending patentapplications describe novel methods for making metal matrix compositematerials (hereinafter sometimes referred to as "Commonly Owned MetalMatrix Patents and Patent Applications").

A novel method of making a metal matrix composite material is disclosedin Commonly Owned U.S. patent application Ser. No. 049,171, filed May13, 1987, in the names of White et al., and entitled "Metal MatrixComposites", now U.S. Pat. No. 4,828,008, which issued May 9, 1989, andwhich published in the EPO on Nov. 17, 1988, as Publication No. 0291441.According to the method of the White et al. invention, a metal matrixcomposite is produced by infiltrating a permeable mass of fillermaterial (e.g., a ceramic or a ceramic-coated material) with moltenaluminum containing at least about 1 percent by weight magnesium, andpreferably at least about 3 percent by weight magnesium. Infiltrationoccurs spontaneously without the application of external pressure orvacuum. A supply of the molten metal alloy is contacted with the mass offiller material at a temperature of at least about 675° C. in thepresence of a gas comprising from about 10 to 100 percent, andpreferably at least about 50 percent, nitrogen by volume, and aremainder of the gas, if any, being a nonoxidizing gas, e.g., argon.Under these conditions, the molten aluminum alloy infiltrates theceramic mass under normal atmospheric pressures to form an aluminum (oraluminum alloy) matrix composite. When the desired amount of fillermaterial has been infiltrated with the molten aluminum alloy, thetemperature is lowered to solidify the alloy, thereby forming a solidmetal matrix structure that embeds the reinforcing filler material.Usually, and preferably, the supply of molten alloy delivered will besufficient to permit the infiltration to proceed essentially to theboundaries of the mass of filler material. The amount of filler materialin the aluminum matrix composites produced according to the White et al.invention may be exceedingly high. In this respect, filler to alloyvolumetric ratios of greater than 1:1 may be achieved.

Under the process conditions in the aforesaid White et al. invention,aluminum nitride can form as a discontinuous phase dispersed throughoutthe aluminum matrix. The amount of nitride in the aluminum matrix mayvary depending on such factors as temperature, alloy composition, gascomposition and filler material. Thus, by controlling one or more suchfactors in the system, it is possible to tailor certain properties ofthe composite. For some end use applications, however, it may bedesirable that the composite contain little or substantially no aluminumnitride.

It has been observed that higher temperatures favor infiltration butrender the process more conducive to nitride formation. The White et al.invention allows the choice of a balance between infiltration kineticsand nitride formation.

An example of suitable barrier means for use with metal matrix compositeformation is described in Commonly Owned U.S. Pat. No. 4,935,055, whichissued on Jun. 19, 1990, from U.S. patent application Ser. No.07/141,642, filed Jan. 7, 1988, in the names of Michael K. Aghajanian etal., and entitled "Method of Making Metal Matrix Composite with the Useof a Barrier", and which published in the EPO on Jul. 12, 1989, asPublication No. 0323945. According to the method of this Aghajanian etal. invention, a barrier means (e.g., particulate titanium diboride or agraphite material such as a flexible graphite tape product sold by UnionCarbide under the trade name GRAFOIL®) is disposed on a defined surfaceboundary of a filler material and matrix alloy infiltrates up to theboundary defined by the barrier means. The barrier means is used toinhibit, prevent, or terminate infiltration of the molten alloy, therebyproviding net, or near net, shapes in the resultant metal matrixcomposite. Accordingly, the formed metal matrix composite bodies have anouter shape which substantially corresponds to the inner shape of thebarrier means.

The method of U.S. Pat. No. 4,828,008 was improved upon by Commonlyowned U.S. Pat. No. 5,298,339, which issued on Mar. 29, 1994, from U.S.patent application Ser. No. 07/994,064, filed Dec. 18, 1992, in thenames of Michael K. Aghajanian and Marc S. Newkirk and entitled "MetalMatrix Composites and Techniques for Making the Same," an equivalentspecification of which published in the EPO on Sep. 20, 1989, asPublication No. 0333629. In accordance with the methods disclosed inthese U.S. Patents and Patent Applications, a matrix metal alloy ispresent as a first source of metal and as a reservoir of matrix metalalloy which communicates with the first source of molten metal due to,for example, gravity flow. Particularly, under the conditions describedin this patent application, the first source of molten matrix alloybegins to infiltrate the mass of filler material under normalatmospheric pressures and thus begins the formation of a metal matrixcomposite. The first source of molten matrix metal alloy is consumedduring its infiltration into the mass of filler material and, ifdesired, can be replenished, preferably by a continuous means, from thereservoir of molten matrix metal as the spontaneous infiltrationcontinues. When a desired amount of permeable filler has beenspontaneously infiltrated by the molten matrix alloy, the temperature islowered to solidify the alloy, thereby forming a solid metal matrixstructure that embeds the reinforcing filler material. It should beunderstood that the use of a reservoir of metal is simply one embodimentof the invention described in this patent application and it is notnecessary to combine the reservoir embodiment with each of the alternateembodiments of the invention disclosed therein, some of which could alsobe beneficial to use in combination with the present invention.

The reservoir of metal can be present in an amount such that it providesfor a sufficient amount of metal to infiltrate the permeable mass offiller material to a predetermined extent. Alternatively, an optionalbarrier means can contact the permeable mass of filler on at least oneside thereof to define a surface boundary.

Moreover, while the supply of molten matrix alloy delivered should be atleast sufficient to permit spontaneous infiltration to proceedessentially to the boundaries (e.g., barriers) of the permeable mass offiller material, the amount of alloy present in the reservoir couldexceed such sufficient amount so that not only will there be asufficient amount of alloy for complete infiltration, but excess moltenmetal alloy could remain and be attached to the metal matrix compositebody (e.g., a macrocomposite). Thus, when excess molten alloy ispresent, the resulting body will be a complex composite body (e.g., amacrocomposite), wherein an infiltrated ceramic body having a metalmatrix therein will be directly bonded to excess metal remaining in thereservoir.

Further improvements in metal matrix technology can be found in CommonlyOwned U.S. Pat. No. 5,249,621, which issued on Oct. 5, 1993, from U.S.patent application Ser. No. 07/863,894, filed Apr. 6, 1992, which was acontinuation of U.S. patent application Ser. No. 07/521,043, filed May9, 1990 (and now abandoned), entitled "A Method of Forming Metal MatrixComposite Bodies by a Spontaneous Infiltration Process, and ProductsProduced Therefrom", which was a Continuation-in-Part application ofU.S. patent application Ser. No. 07/484,753, filed Feb. 23, 1990 (andnow abandoned), which was a Continuation-in-Part application of U.S.patent application Ser. No. 07/432,661, filed Nov. 7, 1989 (and nowabandoned), which was a Continuation-in-Part application of U.S. patentapplication Ser. No. 07/416,327, filed Oct. 6, 1989 (now abandoned),which was a continuation-in-part application of U.S. patent applicationSer. No. 07/349,590, filed May 9, 1989 (and now abandoned), which inturn was a continuation-in-part application of U.S. patent applicationSer. No. 07/269,311, filed Nov. 10, 1988 (now abandoned), all of whichwere filed in the names of Michael K. Aghajanian et al. and all of whichare entitled "A Method of Forming Metal Matrix Composite Bodies By ASpontaneous Infiltration Process, and Products Produced Therefrom" (anEPO application corresponding to U.S. patent application Ser. No.07/416,327 was published in the EPO on Jun. 27, 1990, as EuropeanPublication No. 0 375 558). According to these Aghajanian et al.applications, spontaneous infiltration of a matrix metal into apermeable mass of filler material or preform is achieved by use of aninfiltration enhancer and/or an infiltration enhancer precursor and/oran infiltrating atmosphere which are in communication with the fillermaterial or preform, at least at some point during the process, whichpermits molten matrix metal to spontaneously infiltrate the fillermaterial or preform. Aghajanian et al. disclose a number of matrixmetal/infiltration enhancer precursor/infiltrating atmosphere systemswhich exhibit spontaneous infiltration. Specifically, Aghajanian et al.disclose that spontaneous infiltration behavior has been observed in thealuminum/magnesium/nitrogen system; the aluminum/strontium/nitrogensystem; the aluminum/zinc/oxygen system; and thealuminum/calcium/nitrogen system. However, it is clear from thedisclosure set forth in the Aghajanian et al. applications that thespontaneous infiltration behavior should occur in other matrixmetal/infiltration enhancer precursor/infiltrating atmosphere systems.

Each of the above-discussed Commonly Owned Metal Matrix Patents andPatent Applications describes methods for the production of metal matrixcomposite bodies and novel metal matrix composite bodies which areproduced therefrom. The entire disclosures of all of the foregoingCommonly Owned Metal Matrix Patents and Patent Applications areexpressly incorporated herein by reference.

SUMMARY OF THE INVENTION

A metal matrix composite body is produced by infiltrating a permeablemass of filler material or a preform with a molten matrix metal.Infiltration of the filler material or preform may be carried out by anyconventional liquid metal infiltration technique for forming metalmatrix composites (e.g., pressure infiltration, vacuum infiltration,etc.) or, in a preferred embodiment, by spontaneous infiltration of amolten matrix metal into a filler material or preform in the presence ofan infiltration enhancer, or any combination of these metal matrixcomposite formation techniques. However, prior to such infiltration, agating means is placed between the molten matrix metal and the fillermaterial or preform which is to be infiltrated. The gating meansfunctions to control the areal contact between the molten matrix metaland the permeable filler material or preform. Such control can be usedto deliver molten matrix metal to preferred areas of a preform or fillermaterial; and/or may ameliorate warping of a preform or filler materialby reducing contact between the preform or filler material and matrixmetal carcass; and/or may improve the properties of the resultant metalmatrix composite by providing a sacrificial area (i) for directionalsolidification to occur and (ii) where thermal expansion mismatch can beisolated; and/or may reduce or eliminate the amount of surface machiningrequired to produce a net or near-net shape metal matrix composite.

In a preferred embodiment of the present invention, wherein a metalmatrix composite is formed by spontaneous infiltration, an infiltrationenhancer and/or an infiltration enhancer precursor and/or aninfiltrating atmosphere are in communication with the filler material orpreform, at least at some point during the process, which permits moltenmatrix metal to spontaneously infiltrate the filler material or preform.

In a particularly preferred embodiment of the invention, an infiltrationenhancer may be supplied directly to at least one of the preform orfiller material, and/or matrix metal, and/or infiltrating atmosphere. Inany event, ultimately, at least during spontaneous infiltration, aninfiltration enhancer should be located in at least a portion of thefiller material or preform.

If a gating means is disposed between at least a portion of a moltenmatrix metal and a filler material or preform, an enhanced metal matrixcomposite body can be achieved. Suitable gating means include materialswhich may be permeable or impermeable to molten matrix metal under theprocess conditions; and/or which can facilitate the removal of a carcassof matrix metal which remains in contact with the infiltrated fillermaterial or preform after spontaneous infiltration has been completed.An example of a suitable gating means for use with an aluminum matrixmetal is a graphite material which is substantially impermeable to thematrix metal, such as a graphite tape product, which contains a singlehole or a plurality of holes or passages therethrough. Another exampleof a suitable gating means for use with an aluminum matrix metal is agraphite ring or tube, or a plurality of graphite rings or tubes, whichare also substantially impermeable to the matrix metal but which permitpassage of molten matrix metal therethrough. The ring(s) or tube(s) are,typically, surrounded by a material (e.g., a particulate material), suchas alumina particulate, which, under the process conditions, will not bespontaneously infiltrated by molten matrix metal. Accordingly, thecombination of graphite tube(s) or ring(s) with a surrounding materialwhich is not spontaneously infiltrated limits the areal contact betweenthe molten matrix metal and the filler material or preform to only thepassageway within the graphite ring(s) or tube(s). The number of holesor passageways, size of the holes and passageways and shape of the holesand passageways, etc., can be controlled in any suitable manner so as toachieve an enhanced metal matrix composite. Moreover, the graphite ringor tube may be of any desired cross-sectional geometry (e.g., circular,ovular, polygonal, etc.) and/or any desired vertical geometry (e.g.,straight, tapered, etc.). Moreover, the graphite ring or tube can be ofa sufficient length such that it can function as a riser. Specifically,a first goal of the ring or tube is for it to function as a passagewaywhich permits communication between a source of matrix metal and afiller material or preform. However, a second goal of the riser is tofunction as a sacrificial area where directional solidification voids orporosity can be concentrated. Still further, a third goal of the riseris to ameliorate any thermal stresses which may be present between thematrix metal and the preform which could lead to cracking in the formedmetal matrix composite body. Specifically, thermal stresses can beameliorated by placement of at least some filler material within theriser cavity. Any thermal stresses formed can then be concentratedcloser to the interface between the matrix metal and the riser ratherthan close to the formed metal matrix composite body and the riser. Thefiller material which may be located within the riser cavity maycomprise a material having a composition the same as or different fromthe composition of the permeable mass of filler material or preform.Moreover, the filler material located within the riser cavity may varyin, for example, composition, size, volume percent, etc., from oneportion of the riser cavity to another portion of the riser cavity.

Another example of a suitable impermeable gating means for an aluminummatrix metal is a gating means comprising, e.g., a colloidal graphitelayer, or an aluminum oxide particulate layer which may be applied(e.g., by spraying, etc.) such that it is impermeable to infiltration bymolten aluminum matrix metal, but which contains channels or passagewaystherethrough which permit the molten matrix metal to pass through thegating means and contact the filler material or preform. For example,the impermeable gating means may be provided on a surface of a fillermaterial or preform by first providing a mask or pattern material tocontrol the placement of the gating means on the surface of the fillermaterial or preform to create, for example, a channel or hole, or aplurality of channels or holes, in the gating means. The gating meansmaterial may then be sprayed, dip-coated, silk screened, painted, etc.,onto the surface of the filler material or preform to a thicknesssufficient to render the gating means impermeable to molten matrixmetal. Upon removal of the mask or pattern material, a gating meanslayer having a hole or channel, or a plurality of holes or channels,therein is present on the surface of the filler material or preform.

In a still further embodiment of the invention, a separation means, suchas a relatively thin sheet of metal, which could have a compositionsubstantially similar to or different from the composition of the matrixmetal, could be positioned at a location corresponding to either opening(i.e., the opening located adjacent to the matrix metal or the openinglocated adjacent to the filler material or preform) of the gating means.The separation means should be capable of being at least partiallypermeable to the transport of molten matrix metal at a temperature whichis substantially similar to the melting temperature of the matrix metal(e.g., within about ±100° C.). The separation means facilitates thespontaneous infiltration process.

In another embodiment of the present invention, a gating means which isat least partially permeable to the matrix metal may be provided. Suchpermeable gating means may comprise, for example, a substantiallythree-dimensionally interconnected porous material. Such porous materialcan be formed from a precursor to a porous material. Examples ofsuitable materials for use as permeable gating means which are at leastpartially permeable to an aluminum matrix metal include colloidalgraphite (e.g., DAG® 154 available from Acheson Colloids) and aluminaparticulate (e.g., A-1000 alumina, available from Alcoa IndustriesChemical Division, Bauxite, AR) which may be applied to the surface of afiller material or preform by, for example, spraying, dipping, silkscreening, painting, etc. Without wishing to be bound by any particulartheory or explanation, it is believed that the porous material mayinclude (1) a plurality of microchannels which permit the transport ofmolten matrix metal through the porous material and/or (2) a reactionlayer which is formed under the processing conditions (e.g., formed uponreaction of a precursor to a porous material with at least one of thematrix metal and/or filler material or preform and/or infiltrationenhancer and/or infiltration enhancer precursor and/or infiltratingatmosphere) which reaction layer contains a plurality of microchannelswhich permit the transport of molten matrix metal through the porousmaterial, thus permitting infiltration of the filler material by themolten matrix metal to occur. Moreover, an infiltration enhancerprecursor or an infiltration enhancer may be located at the interfacebetween the matrix metal and the porous material to enhance spontaneousinfiltration of the matrix metal into the filler material or preform.Moreover, it may be possible to modify the composition of the porousmaterial upon contact with molten matrix metal as the matrix metalpasses through the microchannels within the porous material. It isbelieved that upon solidification of the formed metal matrix compositeand the carcass of matrix metal, any stresses which may exist and/orwhich may be created between the metal matrix composite and the carcassof matrix metal (e.g., shear stress, tensile stress, etc., due to, forexample, coefficient of thermal expansion (C.T.E.) mismatch between themetal matrix composite and the carcass of matrix metal, stresses createdduring directional solidification of the metal matrix composite andcarcass of matrix metal, etc.) may facilitate separation of the metalmatrix composite from the carcass of matrix metal. Moreover, it isfurther believed that any reaction layer which may be formed under theprocess conditions may comprise a material which is weak (e.g., possessa fracture strength which is less than the fracture strength of themetal matrix composite or matrix metal carcass) relative to the metalmatrix composite and/or carcass of matrix metal, thereby facilitatingseparation of the metal matrix composite from the carcass of matrixmetal.

The thickness of the permeable gating means which is required to achieveseparation of the metal matrix composite from the carcass of matrixmetal may vary depending upon, for example, the amount and/orcomposition and/or geometry of the matrix metal and/or filler materialor preform and/or gating means, etc., utilized, the reaction conditionsemployed (e.g., time, temperature, etc.), the method or methods used toapply the gating means to the filler material or preform, etc. Forexample, if the gating means layer is not present on the filler materialor preform in a sufficient amount (e.g., the gating means is too thin),the gating means may remain attached to the formed metal matrixcomposite after infiltration has occurred. Moreover, if an excess amountof the gating means is provided to the filler material or preform (e.g.,the gating means is too thick) the matrix metal may not be able topermeate the gating means, thereby inhibiting or preventing infiltrationof the filler material or preform by the matrix metal. Moreover,preferential application of the permeable gating means to the surface ofthe filler material or preform (e.g., varying thickness, varyingcomposition, etc.) may further enhance separation. Thus, without wishingto be bound by any particular theory or explanation, it is believed thata permeable gating means comprising a porous material (which can be madefrom a precursor to a porous material) may be used to control the arealcontact of a molten matrix metal and a filler material or preform and tofacilitate separation depending upon a number of factors including, butnot limited to, the amount and/or composition and/or geometry of thematrix metal and/or filler material or preform, and/or gating meansutilized, the reaction conditions employed, the method of applyingand/or thickness of the gating means, the stresses present between themetal matrix composite and the carcass of matrix metal uponsolidification and the presence of a reaction layer in the porousmaterial.

It is noted that in a preferred embodiment for forming metal matrixcomposites by a spontaneous infiltration technique this applicationdiscusses primarily aluminum matrix metals which, at some point duringthe formation of the metal matrix composite body, are contacted withmagnesium, which functions as the infiltration enhancer precursor, inthe presence of nitrogen, which functions as the infiltratingatmosphere. Thus, the matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system of aluminum/magnesium/nitrogenexhibits spontaneous infiltration. A suitable gating means for use withthis system comprises a graphite material, such as a graphite tapeproduct sold by Union Carbide under the trademark GRAFOIL®. However,other matrix metal/infiltration enhancer precursor/infiltratingatmosphere systems may also behave in a manner similar to the systemaluminum/magnesium/nitrogen. For example, similar spontaneousinfiltration behavior has been observed in thealuminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; andthe aluminum/calcium/nitrogen system. Accordingly, even though thealuminum/magnesium/nitrogen system is discussed primarily herein, itshould be understood that other matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems may behave in a similar mannerand are intended to be encompassed by the invention. A suitable gatingmeans can be chosen for use with each of the other spontaneousinfiltration systems.

When the matrix metal comprises an aluminum alloy, the aluminum alloy iscontacted in a controlled manner with a preform comprising a fillermaterial (e.g., alumina or silicon carbide) or a mass of fillermaterial, said mass of filler material or preform having admixedtherewith, and/or at some point during the process being exposed to,magnesium. Moreover, in a preferred embodiment, the aluminum alloyand/or preform or filler material are contained in a nitrogen atmospherefor at least a portion of the process. The preform will be spontaneouslyinfiltrated and the extent or rate of spontaneous infiltration andformation of metal matrix will vary with a given set of processconditions including, for example, the concentration of magnesiumprovided to the system (e.g., in the aluminum alloy and/or in the fillermaterial or preform and/or in the infiltrating atmosphere), the sizeand/or composition of the particles in the preform or filler material,the concentration of nitrogen in the infiltrating atmosphere, the timepermitted for infiltration, and/or the temperature at which infiltrationoccurs. Spontaneous infiltration typically occurs to an extentsufficient to embed substantially completely the preform or fillermaterial.

Definitions

"Aluminum", as used herein, means and includes essentially pure metal(e.g., a relatively pure, commercially available unalloyed aluminum) orother grades of metal and metal alloys such as the commerciallyavailable metals having impurities and/or alloying constituents such asiron, silicon, copper, magnesium, manganese, chromium, zinc, etc.,therein. An aluminum alloy for purposes of this definition is an alloyor intermetallic compound in which aluminum is the major constituent.

"Balance Non-Oxidizing Gas", as used herein, means that any gas presentin addition to the primary gas comprising the infiltrating atmosphere,is either an inert gas or a reducing gas which is substantiallynon-reactive with the matrix metal under the process conditions. Anyoxidizing gas which may be present as an impurity in the gas(es) usedshould be insufficient to oxidize the matrix metal to any substantialextent under the process conditions.

"Barrier" or "barrier means", as used herein, means any suitable meanswhich interferes with, inhibits, prevents or terminates the migration,movement, or the like, of molten matrix metal beyond a surface boundaryof a permeable mass of filler material or preform, where such surfaceboundary is defined by said barrier means. Suitable barrier means may beany such material, compound, element, composition, or the like, which,under the process conditions, maintains some integrity and is notsubstantially volatile (i.e., the barrier material does not volatilizeto such an extent that it is rendered non-functional as a barrier).

Further, suitable "barrier means" include materials which aresubstantially non-wettable by the migrating molten matrix metal underthe process conditions employed. A barrier of this type appears toexhibit substantially little or no affinity for the molten matrix metal,and movement beyond the defined surface boundary of the mass of fillermaterial or preform is prevented or inhibited by the barrier means. Thebarrier reduces any final machining or grinding that may be required anddefines at least a portion of the surface of the resulting metal matrixcomposite product. The barrier may in certain cases be permeable orporous, or rendered permeable by, for example, drilling holes orpuncturing the barrier, to permit gas to contact the molten matrixmetal.

"Carcass" or "Carcass of Matrix Metal", as used herein, refers to any ofthe original body of matrix metal remaining which has not been consumedduring formation of the metal matrix composite body, and typically, ifallowed to cool, remains in at least partial contact with the metalmatrix composite body which has been formed. It should be understoodthat the carcass may also include a second or foreign metal therein.

"Filler", as used herein, is intended to include either singleconstituents or mixtures of constituents which are substantiallynon-reactive with and/or of limited solubility in the matrix metal andmay be single or multi-phase. Fillers may be provided in a wide varietyof forms, such as powders, flakes, platelets, microspheres, whiskers,bubbles, fibers, particulates, fiber mats, chopped fibers, spheres,pellets, tubules, refractory cloths, etc., and may be either dense orporous. "Filler" may also include ceramic fillers, such as alumina orsilicon carbide as fibers, chopped fibers, particulates, whiskers,bubbles, spheres, fiber mats, or the like, and ceramic-coated fillerssuch as carbon fibers coated with alumina or silicon carbide to protectthe carbon from attack, for example, by a molten aluminum matrix metal.Fillers may also include metals.

"Gating Means", as used herein, means any material or combination ofmaterials which under the process conditions exhibits one or more of thefollowing characteristics: (1) is permeable or substantially impermeableto molten matrix metal relative to the filler material or preform to beinfiltrated; (2) reduces the strength of the bond and/or the amount ofbonding between matrix metal carcass and the infiltrated metal matrixcomposite body, thereby (i) ameliorating the amount of stress (e.g.,warpage) transferred to the metal matrix composite body by the matrixmetal carcass due to differential cooling shrinkage between the carcassof matrix metal and the resultant metal matrix composite body; and/or(ii) reducing the amount of machining required on a surface of aresultant metal matrix composite body due to lessened areal contactbetween the carcass of matrix metal and the resultant metal matrixcomposite body and/or due to lessened areal contact between matrix metaland the preform or filler material which is to be infiltrated, whileinfiltration is occurring.

"Infiltrating Atmosphere", as used herein, means that atmosphere whichis present which interacts with the matrix metal and/or preform (orfiller material) and/or infiltration enhancer precursor and/orinfiltration enhancer which permits or enhances spontaneous infiltrationof the matrix metal.

"Infiltration Enhancer", as used herein, means a material which promotesor assists in the spontaneous infiltration of a matrix metal into afiller material or preform. An infiltration enhancer may be formed from,for example, (1) a reaction of an infiltration enhancer precursor withan infiltrating atmosphere to form a gaseous species and/or (2) areaction product of the infiltration enhancer precursor and theinfiltrating atmosphere and/or (3) a reaction product of theinfiltration enhancer precursor and the filler material or preform.Moreover, the infiltration enhancer may be supplied directly to at leastone of the preform, and/or matrix metal, and/or infiltrating atmosphereand function in a substantially similar manner to an infiltrationenhancer which has formed from a reaction between an infiltrationenhancer precursor and another species. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform to achievespontaneous infiltration, and the infiltration enhancer may be at leastpartially reducible by the matrix metal.

"Infiltration Enhancer Precursor" or "Precursor to the InfiltrationEnhancer", as used herein, means a material which when used incombination (1) with the matrix metal, (2) the preform or fillermaterial and/or (3) an infiltrating atmosphere forms an infiltrationenhancer which induces or assists the matrix metal to spontaneouslyinfiltrate the filler material or preform. Without wishing to be boundby any particular theory or explanation, it appears as though it may benecessary for the precursor to the infiltration enhancer to be capableof being positioned, located or transportable to a location whichpermits the infiltration enhancer precursor to interact with theinfiltrating atmosphere and/or the preform or filler material and/ormetal. For example, in some matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems, it is desirable for theinfiltration enhancer precursor to volatilize at, near, or in somecases, even somewhat above the temperature at which the matrix metalbecomes molten. Such volatilization may lead to: (1) a reaction of theinfiltration enhancer precursor with the infiltrating atmosphere to forma gaseous species which enhances wetting of the filler material orpreform by the matrix metal; and/or (2) a reaction of the infiltrationenhancer precursor with the infiltrating atmosphere to form a solid,liquid or gaseous infiltration enhancer in at least a portion of thefiller material or preform which enhances wetting; and/or (3) a reactionof the infiltration enhancer precursor within the filler material orpreform which forms a solid, liquid or gaseous infiltration enhancer inat least a portion of the filler material or preform which enhanceswetting.

"Matrix Metal" or "Matrix Metal Alloy", as used herein, means that metalwhich is utilized to form a metal matrix composite body (e.g., beforeinfiltration) and/or that metal which is intermingled with a fillermaterial to form a metal matrix composite body (e.g., afterinfiltration). When a specified metal is mentioned as the matrix metal,it should be understood that such matrix metal includes that metal as anessentially pure metal, a commercially available metal having impuritiesand/or alloying constituents therein, an intermetallic compound or analloy in which that metal is the major or predominant constituent.

"Matrix Metal/Infiltration Enhancer Precursor/Infiltrating AtmosphereSystem" or "Spontaneous System", as used herein, refers to thatcombination of materials which exhibit spontaneous metal infiltrationinto a preform or filler material. It should be understood that whenevera "/" appears between an exemplary matrix metal, infiltration enhancerprecursor and infiltrating atmosphere that the "/" is used to designatea system or combination of materials which, when combined in aparticular manner, exhibits spontaneous infiltration into a preform orfiller material.

"Metal Matrix Composite" or "MMC" as used herein, means a materialcomprising a two- or three-dimensionally interconnected alloy or matrixmetal which has embedded a preform or filler material. The matrix metalmay include various alloying elements to provide specifically desiredmechanical and physical properties in the resulting composite.

A Metal "Different" from the Matrix Metal means a metal which does notcontain, as a primary constituent, the same metal as the matrix metal(e.g., if the primary constituent of the matrix metal is aluminum, the"different" metal could have a primary constituent of, for example,nickel).

"Nonreactive Vessel for Housing Matrix Metal" means any vessel which canhouse or contain molten matrix metal and/or filler material (or preform)under the process conditions and not react with the matrix and/or theinfiltrating atmosphere and/or infiltration enhancer precursor and/orfiller material (or preform) in a manner which would be significantlydetrimental to the spontaneous infiltration mechanism.

"Preform" or "Permeable Preform", as used herein, means a porous mass offiller or filler material which is manufactured with at least onesurface boundary which essentially defines a boundary for infiltratingmatrix metal, such mass retaining sufficient shape integrity and greenstrength to provide dimensional fidelity prior to being infiltrated bythe matrix metal. The mass should be sufficiently porous to accommodatespontaneous infiltration of the matrix metal thereinto. A preformtypically comprises a bonded array or arrangement of filler, eitherhomogeneous or heterogeneous, and may be comprised of any suitablematerial (e.g., ceramic and/or metal particulates, powders, fibers,whiskers, etc., and any combination thereof). A preform may exist eithersingularly or as an assemblage.

"Reservoir", as used herein, means a separate body of matrix metalpositioned relative to a mass of filler or a preform so that, when themetal is molten, it may flow to replenish, or in some cases to initiallyprovide and subsequently replenish, that portion, segment or source ofmatrix metal which is in contact with the filler or preform. Thereservoir may also supply at least some metal which is different fromthe matrix metal.

"Riser", as used herein, means both a component of a gating means and agating means per se, which enhances the separation between a metalmatrix composite body and residual matrix metal. The riser issubstantially impermeable to the molten matrix metal and contains atleast one passageway therein which permits the transport of moltenmatrix metal through the riser material. The riser may comprise, forexample, a ring or tube, or any other cylindrical configuration or otherconfiguration which allows transport of molten matrix metaltherethrough.

"Spontaneous Infiltration", as used herein, means the infiltration ofmatrix metal into the permeable mass of filler or preform occurs withoutrequirement for the application of pressure or vacuum (whetherexternally applied or internally created).

BRIEF DESCRIPTION OF DRAWINGS

The following figures are provided to assist in understanding theinvention, but are not intended to limit the scope of the invention.Similar reference numerals have been used wherever possible in each ofthe Figures to denote like components, wherein:

FIG. 1 is a schematic cross section of an assemblage of materialsutilized to form a metal matrix composite body in accordance withExample 1;

FIG. 2 is a schematic cross section of an assemblage material utilizedto form a metal matrix composite body in accordance with Example 2;

FIG. 3 is a photograph which shows both of the infiltrated preform andthe carcass of matrix metal in accordance with Example 1;

FIG. 4 is a photograph which shows both of the infiltrated preform andthe carcass of matrix metal in accordance with Example 2;

FIG. 5 is a schematic cross section of an assemblage of materialsutilized to form a metal matrix composite body in accordance withExample 3;

FIG. 6 is a schematic cross section of an assemblage of materialsutilized to form a metal matrix composite body in accordance withExample 4;

FIG. 7 is a schematic cross section of an assemblage of materialsutilized to form a metal matrix composite body in accordance withExample 7;

FIG. 8 is a schematic cross section of an assemblage of materialsutilized to form a metal matrix composite body in accordance withExample 8;

FIG. 9 is a photograph at 0.3×magnification of a metal matrix compositebody formed in Example 9, Sample A, showing the surface of the bodywhich contacted a barrier means; and

FIG. 10 is a photograph at 0.3×magnification of a formed metal matrixcomposite body formed in Example 9, Sample B, showing the surface whichcontacted a molten matrix metal (without the use of a gating means).

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT

The present invention relates to a method for forming metal matrixcomposites by the use of a gating means to control or regulate theinfiltration of molten matrix metal into a filler material or preform.Infiltration of the filler material or preform may be carried out by anyconventional liquid metal infiltration technique for forming metalmatrix composites (e.g., pressure infiltration, vacuum infiltration,etc.) or, in a preferred embodiment, by spontaneous infiltration of amolten matrix metal into a filler material or preform in the presence ofan infiltration enhancer, or any combination of these metal matrixcomposite formation techniques. Specifically, in the preferredembodiment for forming metal matrix composites by spontaneousinfiltration a molten matrix metal is caused or induced to infiltrate afiller material or a preform after passing through a gating means. Thegating means comprises any material which can be placed between moltenmatrix metal and a permeable filler material or preform and which cancause the molten matrix metal to, preferentially, infiltrate the fillermaterial or preform through at least one passageway defined in thegating means. Thus, the gating means functions to control the arealcontact between the molten matrix metal and the permeable fillermaterial or preform.

Such control can be used to deliver molten matrix metal to preferredareas of a preform or filler material; and/or may ameliorate warping ofa preform or filler material by reducing contact between the preform orfiller material and matrix metal carcass; and/or may improve theproperties of the resultant metal matrix composite by providing asacrificial area (i) for directional solidification to occur and (ii)where thermal expansion mismatch can be isolated; and/or may reduce oreliminate the amount of surface machining required to produce a net ornear-net shape metal matrix composite. Suitable gating means includematerials which typically are impermeable or permeable to molten matrixmetal under the process conditions; and/or which can facilitate theremoval of a carcass of matrix metal which remains in contact with theinfiltrated filler material or preform after spontaneous infiltrationhas been completed.

The passageway(s) provided in the gating means (discussed in greaterdetail later herein), can be of any suitable number or configuration.For example, the gating means may provide a single passageway, aplurality of passageways spaced evenly or randomly apart, or a pluralityof passageways spaced preferentially to enhance formation of a desiredmetal matrix composite configuration. The gating means should be capableof (1) absorbing any cooling stresses which exist between the carcass ofmatrix metal and the formed metal matrix composite and/or (2) reducingthe strength of the bond and/or reducing the amount of bonding betweenthe matrix metal carcass and the formed metal matrix composite. Further,by appropriately locating the passageway(s) in the gating means, thegating means can be utilized to compensate for any uneven infiltrationof matrix metal into the filler material or preform. After properselection, design and placement of the gating means, the permeablepreform or filler material can be infiltrated by molten matrix metal.

In a preferred embodiment of the present invention, wherein a metalmatrix composite is formed by spontaneous infiltration, an infiltrationenhancer and/or an infiltration enhancer precursor and/or aninfiltrating atmosphere are in communication with the filler material orpreform, at least at some point during the process, which permits moltenmatrix metal to spontaneously infiltrate the filler material or preform.

In a particularly preferred embodiment of the invention, an infiltrationenhancer may be supplied directly to at least one of the preform orfiller material, and/or matrix metal, and/or infiltrating atmosphere. Inany event, ultimately, at least during spontaneous infiltration, aninfiltration enhancer should be located in at least a portion of thefiller material or preform.

A gating means for use in the present invention may comprise a layer ofmaterial, e.g., a graphite material or an aluminum oxide material, whichis essentially impermeable (i.e., under the processing conditions moltenmatrix will not infiltrate the layer of material) by the molten matrixmetal, or which is at least partially permeable, and which contains atleast one passageway therethrough which allows molten matrix metal topreferentially, infiltrate the filler material or preform. The gatingmeans may be in the form of, for example, a sheet, a particulate layer,a slurry or paste, or a coating which may be applied to the fillermaterial or preform by, for example, spraying, dipping, silk screening,painting, etc., or any other form or combination of forms whichregulates or controls the location(s) where infiltration of moltenmatrix metal into a filler material or preform can occur. An example ofa suitable gating means for use with an aluminum matrix metal is agraphite material which is substantially impermeable to the matrixmetal, such as a graphite tape product which contains a single hole or aplurality of holes or passages therethrough. In a particularly preferredembodiment, the gating means comprises a graphite tape product soldunder the trademark GRAFOIL®, registered to Union Carbide, havingtherein at least one passageway which permits the transport of moltenmatrix metal preferentially therethrough. Upon infiltration of thefiller material or preform by the molten matrix metal, the formed metalmatrix composite is attached to the residual matrix metal only at thepassageway(s) within the gating means. The separation of the formedmetal matrix composite body from the residual matrix metal isfacilitated, thereby minimizing the need for time-consuming and/orexpensive machining of the metal matrix composite body to remove excessmatrix metal.

In a further preferred embodiment of the present invention, the gatingmeans may include a material or a combination of materials which enhancethe separation between a formed metal matrix composite body and residualmatrix metal. For example, the gating means may comprise at least oneriser, e.g., a ring or a tube of material which is substantiallyimpermeable to by the molten matrix metal under the processingconditions and which provides at least one passageway between the moltenmatrix metal and the filler material or preform. An example of asuitable gating means for use with an aluminum matrix metal is agraphite ring or tube, or a plurality of rings or tubes, which aresubstantially impermeable to the matrix metal but which permit passageof molten matrix metal therethrough. The gating means is located betweena filler material or preform and a source of molten matrix metal bypositioning at least one riser such that it provides a passagewaybetween the filler material or preform and the source of molten matrixmetal. Steps must be taken to assure that matrix metal is not permittedto flow into any remaining space between the preform or filler materialand the source of matrix metal. For example, the remaining space couldbe filled with a powder or particulate material, such as an aluminaparticulate, which is substantially non-wettable by the molten matrixmetal under the process conditions. Alternatively, the contact areabetween the preform or filler material and the gating means could beappropriately sealed to prevent molten matrix metal from contacting thepreform or filler material at any location other than the gating means.Thus, under the processing conditions, molten matrix metalpreferentially, spontaneously infiltrates the filler material or preformthrough the passageway(s) established by the riser(s) in the gatingmeans. The number of holes or passageways, size of the holes andpassageways and shape of the holes and passageways, etc., can becontrolled in any suitable manner so as to achieve an enhanced metalmatrix composite. Moreover, when using a graphite ring or tube, thegraphite ring or tube may be of any desired cross-sectional geometry(e.g., circular, ovular, polygonal, etc.) and/or any desired verticalgeometry (e.g., straight, tapered, etc.). Moreover, the graphite ring ortube can be of a sufficient length such that it can function as a riser.Specifically, a first goal of the ring or tube is for it to function asa passageway which permits communication between a source of matrixmetal and a filler material or preform.

In a particularly preferred embodiment, the gating means may comprise atleast one riser, comprising, for example, a graphite ring or tube, whichspatially separates the source of matrix metal and the preform or fillermaterial. A particulate material comprising, for example, an aluminaparticulate material which is non-wettable by the molten matrix metalunder the processing conditions, is then placed into the open spacewhich exists between the source of matrix metal and the preform orfiller material, thereby substantially completely surrounding the riser.Upon infiltration of the filler material or preform by the molten matrixmetal, the uninfiltrated powder or particulate material of the gatingmeans, typically, is readily removed by only minimal mechanicalagitation (e.g., pouring it out), thus leaving the riser portion(s) ofthe gating means as the only point(s) of attachment between the metalmatrix composite and the residual matrix metal. Thus, after infiltrationis completed, the riser portion may be readily removed with minimalmachining (e.g., cutting or sectioning the riser with a saw, etc.).

To improve further the mechanical and/or physical properties of metalmatrix composite bodies, directional solidification of the formed metalmatrix composite body can be effected. For example, by utilizing thedirectional solidification techniques discussed in commonly owned U.S.Pat. No. 5,303,763, which issued on Apr. 19, 1994 in the names ofMichael K. Aghajanian et al. and entitled "Directional Solidification ofMetal Matrix Composites", shrinkage voids can be substantiallyeliminated. For example, by placing a formed metal matrix composite bodyproduced by the method of the present invention onto a chill plate(e.g., a water-cooled aluminum quench plate), solidification porosityand/or flaws within the composite body may be reduced or substantiallyeliminated. Moreover, a hot topping material may be provided on the topof the residual matrix metal to enhance directional solidification ofthe formed metal matrix composite body, thus further reducing oreliminating solidification defects in the composite body.

A second goal of the riser is to provide a sacrificial area wheredirectional solidification voids or porosity can be concentrated. Theresidual matrix metal within the riser of the gating means can be usedas a sacrificial section to further improve the properties of the formedmetal matrix composite body. Specifically, after infiltration hasoccurred, directional solidification of the formed metal matrixcomposite body can be effected and any solidification porosity whichtypically forms can be substantially eliminated from the composite bodyby causing such porosity to be substantially completely concentrated inthe riser of the gating means and/or in the carcass of matrix metal. Theriser containing residual matrix metal may then be easily removed fromthe formed metal matrix composite body.

The riser can also be constructed to ameliorate other flaws which mayexist or be created in the metal matrix composite body, which is a thirdgoal of the riser. For example, the coefficient of thermal expansion(CTE) of the formed metal matrix composite body (i.e., matrix metal plusfiller) may differ from the CTE of the residual matrix metal (i.e.,matrix metal only). Thus, by utilizing the riser to physically separatethe matrix metal from the formed metal matrix composite body, crackswhich may form in the composite body during heat-up and/or cool-down maybe substantially reduced or eliminated. Moreover, the use of a riseralone may not completely ameliorate all flaws or cracks in the formedmetal matrix composite body. Accordingly, it may be necessary to utilizea filler material within the riser of the gating means prior toinfiltrating the filler material or preform with molten matrix metal.The use of a filler material in the riser effectively moves the point ofCTE mismatch away from the surface of the metal matrix composite bodyand into the sacrificial metal matrix portion (i.e., that portion of theriser having filler material infiltrated by matrix metal) within theriser. The sacrificial metal matrix within the riser can then be removed(e.g., by machining) from the formed metal matrix composite body withoutthe occurrence of any flaws in the formed composite body. Moreover,depending on the desired result, the filler material provided within thesacrificial portion may be the same or different in size and/or shapeand/or filler loading and/or composition than the filler in the formedmetal matrix composite body. For example, by providing a filler materialwithin the sacrificial portion which is larger in size than the fillerin the formed metal matrix composite body, the flow of matrix metal intothe filler material or preform during infiltration will not besubstantially reduced.

Another example of a suitable impermeable gating means for an aluminummatrix metal is a gating means comprising, e.g., a colloidal graphitelayer, or an aluminum oxide particulate layer which may be applied(e.g., by spraying, etc.) such that it is impermeable to infiltration bymolten aluminum matrix metal, but which contains channels or passagewaystherethrough which permit the molten matrix metal to pass through thegating means and contact the filler material or preform. For example,the impermeable gating means may be provided on a surface of a fillermaterial or preform by first providing a mask or pattern material tocontrol the placement of the gating means on the surface of the fillermaterial or preform to create, for example, a channel or hole, or aplurality of channels or holes, in the gating means. The gating meansmaterial may then be sprayed, dip-coated, silk screened, painted, etc.,onto the surface of the filler material or preform to a thicknesssufficient to render the gating means impermeable to molten matrixmetal. Upon removal of the mask or pattern material, a gating meanslayer having a hole or channel, or a plurality of holes or channels,therein is present on the surface of the filler material or preform.

In a further embodiment of the present invention, a separation means,e.g., a metal foil or a reducible oxide, nitride, etc., may be providedto at least a portion of an interface between, for example, the matrixmetal and the gating means or the gating means and the formed metalmatrix composite, to enhance formation of a metal matrix composite bodyby the method of the present invention. The separation means may beutilized to facilitate separation of the formed body and the residualmatrix metal by reacting with, for example, the matrix metal and/or theinfiltrating atmosphere, to form a reaction product (e.g., anintermetallic, a nitride, etc.) which may exhibit, for example,brittleness or CTE mismatch. For example, for an aluminum matrix metal,a suitable separation means comprises an aluminum metal foil which, uponcontact with an infiltrating atmosphere of nitrogen, forms aluminumnitride which is more brittle than the matrix metal, thus facilitatingseparation of, for example, the metal matrix composite from the matrixmetal. The composition of the separation means may be substantially thesame as or different from the composition of the matrix metal, and theseparation means may be solid or perforated. Moreover, the separationmeans does not substantially interfere with the transport of moltenmatrix metal into the filler material or preform. For example, theseparation means, typically, is not impermeable to the flow of moltenmatrix metal. Further, it is believed that the separation means mayenhance the formation of metal matrix composite bodies by preventinginfiltration enhancer precursor and/or infiltration enhancer, uponvaporization, when vaporization is utilized, from escaping from thereaction system, thus facilitating the coating of filler material withinfiltration enhancer.

In another embodiment of the present invention, a gating means which isat least partially permeable to the matrix metal may be provided. Suchpermeable gating means may comprise, for example, a substantiallythree-dimensionally interconnected porous material. Such porous materialcan be formed from a precursor to a porous material. Examples ofsuitable materials for use as permeable gating means which are at leastpartially permeable to an aluminum matrix metal include colloidalgraphite (e.g., DAG® 154 available from Acheson Colloids) and aluminaparticulate (e.g., A-1000 alumina, available from Alcoa IndustriesChemical Division, Bauxite, AR) which may be applied to the surface of afiller material or preform by, for example, spraying, dipping, silkscreening, painting, etc. Without wishing to be bound by any particulartheory or explanation, it is believed that the porous material mayinclude (1) a plurality of microchannels which permit the transport ofmolten matrix metal through the porous material and/or (2) a reactionlayer which is formed under the processing conditions (e.g., formed uponreaction of a precursor to a porous material with at least one of thematrix metal and/or filler material or preform and/or infiltrationenhancer and/or infiltration enhancer precursor and/or infiltratingatmosphere) which reaction layer contains a plurality of microchannelswhich permit the transport of molten matrix metal through the porousmaterial, thus permitting infiltration of the filler material by themolten matrix metal to occur. Moreover, an infiltration enhancerprecursor or an infiltration enhancer may be located at the interfacebetween the matrix metal and the porous material to enhance spontaneousinfiltration of the matrix metal into the filler material or preform.Moreover, it may be possible to modify the composition of the porouslayer upon contact with molten matrix metal as matrix metal passesthrough the microchannels within the porous material. It is believedthat upon solidification of the formed metal matrix composite and thecarcass of matrix metal, any stresses which may exist and/or which maybe created between the metal matrix composite and the carcass of matrixmetal (e.g., shear stress, tensile stress, etc., due to, for example,coefficient of thermal expansion (C.T.E.) mismatch between the metalmatrix composite and the carcass of matrix metal, stresses createdduring directional solidification of the metal matrix composite andcarcass of matrix metal, etc.) may facilitate separation of the metalmatrix composite from the carcass of matrix metal. Moreover, it isfurther believed that any reaction layer which may be formed under theprocess conditions may comprise a material which is weak (e.g., possessa fracture strength which is less than the fracture strength of themetal matrix composite or matrix metal carcass) relative to the metalmatrix composite and/or carcass of matrix metal, thereby facilitatingseparation of the metal matrix composite from the carcass of matrixmetal.

The thickness of the permeable gating means which is required to achieveseparation of the metal matrix composite from the carcass of matrixmetal may vary depending upon, for example, the amount and/orcomposition and/or geometry of the matrix metal and/or filler materialor preform and/or gating means, etc., utilized, the reaction conditionsemployed (e.g., time, temperature, etc.), the method or methods used toapply the gating means to the filler material or preform, etc. Forexample, if the gating means layer is not present on the filler materialor preform in a sufficient amount (e.g., the gating means is too thin),the gating means may remain attached to the formed metal matrixcomposite after infiltration has occurred. Moreover, if an excess amountof the gating means is provided to the filler material or preform (e.g.,the gating means is too thick) the matrix metal may not be able topermeate the gating means, thereby inhibiting or preventing infiltrationof the filler material or preform by the matrix metal. Moreover,preferential application of the permeable gating means to the surface ofthe filler material or preform (e.g., varying thickness, varyingcomposition, etc.) may further enhance separation. Thus, without wishingto be bound by any particular theory or explanation, it is believed thata permeable gating means comprising a porous material (which can be madefrom a precursor to a porous material) may be used to control the arealcontact of a molten matrix metal and a filler material or preform and tofacilitate separation depending upon a number of factors including, butnot limited to, the amount and/or composition and/or geometry of thematrix metal and/or filler material or preform, and/or gating meansutilized, the reaction conditions employed, the method of applyingand/or thickness of the gating means, the stresses present between themetal matrix composite and the carcass of matrix metal uponsolidification and the presence of a reaction layer in the porousmaterial.

Depending on the desired geometry of the formed metal matrix compositebody and any final machining capabilities, the gating means may beprovided in any number of desired configurations. For example, a gatingmeans may be provided on the top or bottom of the filler material orpreform, or at the sides of the filler material or preform, or anycombination thereof. Moreover, the preform or filler material may beconfigured so as to enhance the formation of a metal matrix compositebody by the use of a gating means. For example, a portion of a preformor filler material which contacts the gating means may be configured soas to become sacrificial and easily removed from the final metal matrixcomposite body.

In a preferred embodiment, filler material or preform is configured suchthat a sacrificial section extends beyond the geometry of the finalcomposite body to be manufactured. For example, a projection or tabwhich is, e.g., rectangular, in shape and which may extend from only asmall portion or, optionally, the entire length or width of a surface ofthe preform or filler material, may be provided as the contact surfacefor the gating means in the method of the present invention. The tabportion may contain a filler material which is of a composition and/orsize which is identical to, similar to or different from, the fillermaterial in the remaining portion of the preform or filler material.After infiltration of the preform or filler material by molten matrixmetal, the tab may be easily machined off of the final composite body,thus facilitating production of flaw-free, near net-shape metal matrixcomposite bodies. Moreover, when infiltrating a loose mass of fillermaterial, by providing a sacrificial tab section in the mold orretaining bed for contact with the gating means, the surface of theloose mass of filler material may be controlled (e.g., weighted using aweighting means) to retain the loose mass in a substantially uniformconfiguration, thus minimizing shifting of the loose filler uponinfiltration.

To achieve infiltration of a filler material or preform by the preferredspontaneous infiltration technique, an infiltration enhancer and/orinfiltration enhancer precursor and/or infiltrating atmosphere are incommunication with the filler material or preform, at least at somepoint during the process, which permits molten matrix metal tospontaneously infiltrate the filler material or preform through thegating means. Specifically, in order to effect spontaneous infiltrationof the matrix metal into the filler material or preform, an infiltrationenhancer should be provided to the spontaneous system. An infiltrationenhancer could be formed from an infiltration enhancer precursor whichcould be provided (1) in the matrix metal; and/or (2) in the fillermaterial or preform; and/or (3) from the infiltrating atmosphere and/or(4) from an external source into the spontaneous system. Moreover,rather than supplying an infiltration enhancer precursor, aninfiltration enhancer may be supplied directly to at least one of thefiller material or preform, and/or matrix metal, and/or infiltratingatmosphere. Ultimately, at least during the spontaneous infiltration,the infiltration enhancer should be located in at least a portion of thefiller material or preform.

In a further preferred embodiment for forming metal matrix composites byspontaneous infiltration it is possible that the infiltration enhancerprecursor can be at least partially reacted with the infiltratingatmosphere such that infiltration enhancer can be formed in at least aportion of the filler material or preform prior to or substantiallysimultaneously with contacting the preform or filler material withmolten matrix metal (e.g., if magnesium was the infiltration enhancerprecursor and nitrogen was the infiltrating atmosphere, the infiltrationenhancer could be magnesium nitride which would be located in at least aportion of the filler material or preform).

An example of a matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system is thealuminum/magnesium/nitrogen system. Specifically, an aluminum matrixmetal can be embedded within a filler material which can be containedwithin a suitable refractory vessel which, under the process conditions,does not react with the aluminum matrix metal and/or the filler materialwhen the aluminum is made molten. A filler material containing or beingexposed to magnesium, and being exposed to, at least at some pointduring the processing, a nitrogen atmosphere, can be contacted with themolten aluminum matrix metal. The matrix metal will then spontaneouslyinfiltrate the filler material or preform.

Moreover, rather than supplying an infiltration enhancer precursor, aninfiltration enhancer may be supplied directly to at least one of thepreform, and/or matrix metal, and/or infiltrating atmosphere.Ultimately, at least during the spontaneous infiltration, theinfiltration enhancer should be located in at least a portion of thefiller material or preform.

Under the conditions employed in the method of the present invention, inthe case of an aluminum/magnesium/ nitrogen spontaneous infiltrationsystem, the filler material or preform should be sufficiently permeableto permit the nitrogen-containing gas to penetrate or permeate thefiller material or preform at some point during the process and/orcontact the molten matrix metal. Moreover, the permeable filler materialor preform can accommodate infiltration of the molten matrix metal,thereby causing the nitrogen-permeated filler material or preform to beinfiltrated spontaneously with molten matrix metal to form a metalmatrix composite body and/or cause the nitrogen to react with aninfiltration enhancer precursor to form infiltration enhancer in thefiller material or preform and thereby resulting in spontaneousinfiltration. The extent or rate of spontaneous infiltration andformation of the metal matrix composite will vary with a given set ofprocess conditions, including magnesium content of the aluminum alloy,magnesium content of the filler material or preform, amount of magnesiumnitride in the filler material or preform, the presence of additionalalloying elements (e.g., silicon, iron, copper, manganese, chromium,zinc, and the like), average size of the filler material (e.g., particlediameter), surface condition and type of filler material, nitrogenconcentration of the infiltrating atmosphere, time permitted forinfiltration and temperature at which infiltration occurs. For example,for infiltration of the molten aluminum matrix metal to occurspontaneously, the aluminum can be alloyed with at least about 1% byweight, and preferably at least about 3% by weight, magnesium (whichfunctions as the infiltration enhancer precursor), based on alloyweight. Auxiliary alloying elements, as discussed above, may also beincluded in the matrix metal to tailor specific properties thereof.Additionally, the auxiliary alloying elements may affect the minimumamount of magnesium required in the matrix aluminum metal to result inspontaneous infiltration of the filler material or preform. Loss ofmagnesium from the spontaneous system due to, for example,volatilization should not occur to such an extent that no magnesium waspresent to form infiltration enhancer. Thus, it is desirable to utilizea sufficient amount of initial alloying elements to assure thatspontaneous infiltration will not be adversely affected byvolatilization. Still further, the presence of magnesium in both of thefiller material or preform and matrix metal or the filler material orpreform alone may result in a reduction in the required amount ofmagnesium to achieve spontaneous infiltration (discussed in greaterdetail later herein).

The volume percent of nitrogen in the nitrogen atmosphere also affectsformation rates of the metal matrix composite body. Specifically, ifless than about 10 volume percent of nitrogen is present in theatmosphere, very slow or little spontaneous infiltration will occur. Ithas been discovered that it is preferable for at least about 50 volumepercent of nitrogen to be present in the infiltrating atmosphere,thereby resulting in, for example, shorter infiltration times due to amuch more rapid rate of infiltration. The infiltrating atmosphere (e.g.,a nitrogen-containing gas) can be supplied directly to the fillermaterial or preform and/or matrix metal, or it may be produced or resultfrom a decomposition of a material.

The minimum magnesium content required for molten matrix metal toinfiltrate a filler material or preform depends on one or more variablessuch as the processing temperature, time, the presence of auxiliaryalloying elements such as silicon or zinc, the nature of the fillermaterial, the location of the magnesium in one or more components of thespontaneous system, the nitrogen content of the atmosphere, and the rateat which the nitrogen atmosphere flows. Lower temperatures or shorterheating times can be used to obtain complete infiltration as themagnesium content of the alloy and/or preform is increased. Also, for agiven magnesium content, the addition of certain auxiliary alloyingelements such as zinc permits the use of lower temperatures. Forexample, a magnesium content of the matrix metal at the lower end of theoperable range, e.g., from about 1 to 3 weight percent, may be used inconjunction with at least one of the following: an above-minimumprocessing temperature, a high nitrogen concentration, or one or moreauxiliary alloying elements. When no magnesium is added to the fillermaterial or preform, alloys containing from about 3 to 5 weight percentmagnesium are preferred on the basis of their general utility over awide variety of process conditions, with at least about 5 percent beingpreferred when lower temperatures and shorter times are employed.Magnesium contents in excess of about 10 percent by weight of thealuminum alloy may be employed to moderate the temperature conditionsrequired for infiltration. The magnesium content may be reduced whenused in conjunction with an auxiliary alloying element, but theseelements serve an auxiliary function only and are used together with atleast the above-specified minimum amount of magnesium. For example,there was substantially no infiltration of nominally pure aluminumalloyed only with 10 percent silicon at 1000° C. into a bedding of 500mesh, 39 CRYSTOLON® (99 percent pure silicon carbide from Norton Co.).However, in the presence of magnesium, silicon has been found to promotethe infiltration process. As a further example, the amount of magnesiumvaries if it is supplied exclusively to the preform or filler material.It has been discovered that spontaneous infiltration will occur with alesser weight percent of magnesium supplied to the spontaneous systemwhen at least some of the total amount of magnesium supplied is placedin the preform or filler material. It may be desirable for a lesseramount of magnesium to be provided in order to prevent the formation ofundesirable intermetallics in the metal matrix composite body. In thecase of a silicon carbide preform, it has been discovered that when thepreform is contacted with an aluminum matrix metal, the preformcontaining at least about 1% by weight magnesium and being in thepresence of a substantially pure nitrogen atmosphere, the matrix metalspontaneously infiltrates the preform. In the case of an aluminapreform, the amount of magnesium required to achieve acceptablespontaneous infiltration is slightly higher. Specifically, it has beenfound that when an alumina preform, when contacted with a similaraluminum matrix metal, at about the same temperature as the aluminumthat infiltrated into the silicon carbide preform, and in the presenceof the same pure nitrogen atmosphere, at least about 3% by weightmagnesium may be required to achieve similar spontaneous infiltration tothat achieved in the silicon carbide preform discussed immediatelyabove.

It is also noted that it is possible to supply to the spontaneous systeminfiltration enhancer precursor and/or infiltration enhancer on asurface of the alloy and/or on a surface of the preform or fillermaterial and/or within the preform or filler material prior toinfiltrating the matrix metal into the filler material or preform (i.e.,it may not be necessary for the supplied infiltration enhancer orinfiltration enhancer precursor to be alloyed with the matrix metal, butrather, simply supplied to the spontaneous system). If the magnesium wasapplied to a surface of the matrix metal it may be preferred that saidsurface should be the surface which is closest to, or preferably incontact with, the permeable mass of filler material or vice versa; orsuch magnesium could be mixed into at least a portion of the preform orfiller material. Still further, it is possible that some combination ofsurface application, alloying and placement of magnesium into at least aportion of the filler material could be used. Such combination ofapplying infiltration enhancer(s) and/or infiltration enhancerprecursor(s) could result in a decrease in the total weight percent ofmagnesium needed to promote infiltration of the matrix aluminum metalinto the filler material, as well as achieving lower temperatures atwhich infiltration can occur. Moreover, the amount of undesirableintermetallics formed due to the presence of magnesium could also beminimized.

The use of one or more auxiliary alloying elements and the concentrationof nitrogen in the surrounding gas also affects the extent of nitridingof the matrix metal at a given temperature. For example, auxiliaryalloying elements such as zinc or iron included in the alloy, or placedon a surface of the alloy, may be used to reduce the infiltrationtemperature and thereby decrease the amount of nitride formation,whereas increasing the concentration of nitrogen in the gas may be usedto promote nitride formation.

The concentration of magnesium in the alloy, and/or placed onto asurface of the alloy, and/or combined in the filler or preform material,also tends to affect the extent of infiltration at a given temperature.Consequently, in some cases where little or no magnesium is contacteddirectly with the preform or filler material, it may be preferred thatat least about three weight percent magnesium be included in the alloy.Alloy contents of less than this amount, such as one weight percentmagnesium, may require higher process temperatures or an auxiliaryalloying element for infiltration. The temperature required to effectthe spontaneous infiltration process of this invention may be lower: (1)when the magnesium content of the alloy alone is increased, e.g. to atleast about 5 weight percent; and/or (2) when alloying constituents aremixed with the permeable mass of filler material or preform; and/or (3)when another element such as zinc or iron is present in the aluminumalloy. The temperature also may vary with different filler materials. Ingeneral, spontaneous and progressive infiltration will occur at aprocess temperature of at least about 675° C., and preferably a processtemperature of at least about 750° C.-800° C. Temperatures generally inexcess of 1200° C. do not appear to benefit the process, and aparticularly useful temperature range has been found to be from about675° C. to about 1200° C. However, as a general rule, the spontaneousinfiltration temperature is a temperature which is above the meltingpoint of the matrix metal but below the volatilization temperature ofthe matrix metal. Moreover, the spontaneous infiltration temperatureshould be below the melting point of the filler material. Still further,as temperature is increased, the tendency to form a reaction productbetween the matrix metal and infiltrating atmosphere increases (e.g., inthe case of aluminum matrix metal and a nitrogen infiltratingatmosphere, aluminum nitride may be formed). Such reaction product maybe desirable or undesirable, dependent upon the intended application ofthe metal matrix composite body. Typically, electric resistance heatingis utilized to achieve the infiltration temperature. However, anyheating means which can cause the matrix metal to become molten and doesnot adversely affect spontaneous infiltration is acceptable for use withthe invention.

In the present method, for example, a permeable filler material orpreform comes into contact with molten aluminum in the presence of, atleast sometime during the process, a nitrogen-containing gas. Thenitrogen-containing gas may be supplied by maintaining a continuous flowof gas into contact with at least one of the filler material or thepreform and/or molten aluminum matrix metal. Although the flow rate ofthe nitrogen-containing gas is not critical, it is preferred that theflow rate be sufficient to compensate for any nitrogen lost from theatmosphere due to nitride formation in the alloy matrix, and also toprevent or inhibit the incursion of air which can have an oxidizingeffect on the molten metal.

The methods of forming metal matrix composites in the present inventionare applicable to a wide variety of filler materials, and the choice offiller materials will depend on such factors as the matrix alloy, theprocess conditions, the reactivity of the molten matrix alloy with thefiller material, the ability of the filler material to conform to thematrix metal, and the properties sought for the final composite product.For example, when aluminum is the matrix metal, suitable fillermaterials include (a) oxides, e.g. alumina; (b) carbides, e.g. siliconcarbide; (c) borides, e.g. aluminum dodecaboride, and (d) nitrides, e.g.aluminum nitride and mixtures thereof. If there is a tendency for thefiller material to react with the molten aluminum matrix metal, thismight be accommodated by minimizing the infiltration time andtemperature or by providing a non-reactive coating on the filler. Thefiller material may comprise a substrate, such as carbon or othernon-ceramic material, bearing a coating to protect the substrate fromattack or degradation. Suitable ceramic coatings include oxides,carbides, borides and nitrides. Ceramics which are preferred for use inthe present method include alumina and silicon carbide in the form ofparticles, platelets, whiskers and fibers. The fibers can bediscontinuous (in chopped form) or in the form of a woven mat and acontinuous filament, such as multifilament tows. Further, the fillermaterial may be homogeneous or heterogeneous.

Certain filler materials exhibit enhanced infiltration relative tofiller materials having a similar chemical composition. For example,crushed alumina bodies made by the method disclosed in U.S. Pat. No.4,713,360, entitled "Novel Ceramic Materials and Methods of MakingSame", which issued on Dec. 15, 1987, in the names of Marc S. Newkirk etal., exhibit desirable infiltration properties relative to commerciallyavailable alumina products. Moreover, crushed alumina bodies made by themethod disclosed in Commonly Owned U.S. Pat. No. 4,851,375 entitled"Methods of Making Composite Ceramic Articles Having Embedded Filler",which issued Jul. 25, 1989, in the names of Marc S. Newkirk et al., alsoexhibit desirable infiltration properties relative to commerciallyavailable alumina products. The subject matter of each of the issuedpatents is herein expressly incorporated by reference. Specifically, ithas been discovered that complete infiltration of a permeable mass of aceramic or ceramic composite material can occur at lower infiltrationtemperatures and/or lower infiltration times by utilizing a crushed orcomminuted body produced by the method of the aforementioned U.S. Patentand patent application.

The size and shape of the filler material can be any that may berequired to achieve the properties desired in the composite and whichcan conform to the matrix metal. Thus, the filler material may be in theform of particles, whiskers, platelets, fibers or mixtures sinceinfiltration is not restricted by the shape of the filler material.Other shapes such as spheres, tubules, pellets, refractory fiber cloth,and the like may be employed. In addition, the size of the material doesnot limit infiltration, although a higher temperature or longer timeperiod may be needed for complete infiltration of a mass of smallerparticles than for larger particles. Further, the mass of fillermaterial (shaped into a preform) to be infiltrated should be permeable,i.e., permeable to molten matrix metal and to the infiltratingatmosphere.

The method of forming metal matrix composites according to the presentinvention, not being dependent on the use of pressure to force ordisplace molten matrix metal into a preform or a mass of fillermaterial, permits the production of substantially uniform metal matrixcomposites having a high volume fraction of filler material and lowporosity. Higher volume fractions of filler material on the order of atleast about 50% may be achieved by using a lower porosity initial massof filler material and mixtures of particle sizes and by admixingparticles of varying size. Higher volume fractions also may be achievedif the mass of filler is compacted or otherwise densified provided thatthe mass is not converted into either a compact with close cell porosityor into a fully dense structure that would prevent infiltration by themolten alloy.

It has been observed that for aluminum infiltration and matrix formationaround a ceramic filler, wetting of the ceramic filler by the aluminummatrix metal may be an important part of the infiltration mechanism.Moreover, at low processing temperatures, a negligible or minimal amountof metal nitriding occurs resulting in a minimal discontinuous phase ofaluminum nitride dispersed in the metal matrix. However, as the upperend of the temperature range is approached, nitridation of the metal ismore likely to occur. Thus, the amount of the nitride phase in the metalmatrix can be controlled by varying the processing temperature at whichinfiltration occurs. The specific process temperature at which nitrideformation becomes more pronounced also varies with such factors as thematrix aluminum alloy used and its quantity relative to the volume offiller material, the filler material to be Infiltrated, and the nitrogenconcentration of the infiltrating atmosphere. For example, the extent ofaluminum nitride formation at a given process temperature is believed toincrease as the ability of the alloy to wet the filler decreases and asthe nitrogen concentration of the atmosphere increases.

It is therefore possible to tailor the constituency of the metal matrixduring formation of the composite to impart certain characteristics tothe resulting product. For a given system, the process conditions can beselected to control the nitride formation. A composite productcontaining an aluminum nitride phase will exhibit certain propertieswhich can be favorable to, or improve the performance of, the product.Further, the temperature range for spontaneous infiltration with analuminum alloy may vary with the ceramic material used. In the case ofalumina as the filler material, the temperature for infiltration shouldpreferably not exceed about 1000° C. if it is desired that the ductilityof the matrix not be reduced by the significant formation of nitride.However, temperatures exceeding 1000° C. may be employed if it isdesired to produce a composite with a less ductile and stiffer matrix.To infiltrate silicon carbide, higher temperatures of about 1200° C. maybe employed since the aluminum alloy nitrides to a lesser extent,relative to the use of alumina as filler, when silicon carbide isemployed as a filler material.

Moreover, it is possible to use a reservoir of matrix metal to assurecomplete infiltration of the filler material and/or to supply a secondmetal which has a different composition from the first source of matrixmetal. Specifically, in some cases it may be desirable to utilize amatrix metal in the reservoir which differs in composition from thefirst source of matrix metal. For example, if an aluminum alloy is usedas the first source of matrix metal, then virtually any other metal ormetal alloy which was molten at the processing temperature could be usedas the reservoir metal. Molten metals frequently are very miscible witheach other which would result in the reservoir metal mixing with thefirst source of matrix metal so long as an adequate amount of time isgiven for the mixing to occur. Thus, by using a reservoir metal which isdifferent in composition than the first source of matrix metal, it ispossible to tailor the properties of the metal matrix to meet variousoperating requirements and thus tailor the properties of the metalmatrix composite.

A barrier means may also be utilized in combination with the presentinvention. Specifically, the barrier means for use with this inventionmay be any suitable means which interferes, inhibits, prevents orterminates the migration, movement, or the like, of molten matrix alloy(e.g., an aluminum alloy) beyond the defined surface boundary of thefiller material. Suitable barrier means may be any material, compound,element, composition, or the like, which, under the process conditionsof this invention, maintains some integrity, is not volatile andpreferably is permeable to the gas used with the process as well asbeing capable of locally inhibiting, stopping, interfering with,preventing, or the like, continued infiltration or any other kind ofmovement beyond the defined surface boundary of the filler material.

Suitable barrier means includes materials which are substantiallynon-wettable by the migrating molten matrix alloy under the processconditions employed. A barrier of this type appears to exhibit little orno affinity for the molten matrix alloy, and movement beyond the definedsurface boundary of the filler material is prevented or inhibited by thebarrier means. The barrier reduces any final machining or grinding thatmay be required of the metal matrix composite product. As stated above,the barrier preferably should be permeable or porous, or renderedpermeable by puncturing, to permit the gas to contact the molten matrixalloy.

Suitable barriers particularly useful for aluminum matrix alloys arethose containing carbon, especially the crystalline allotropic form ofcarbon known as graphite. Graphite is essentially non-wettable by themolten aluminum alloy under the described process conditions. Aparticularly preferred graphite is a graphite tape product that is soldunder the trademark GRAFOIL®, registered to Union Carbide. This graphitetape exhibits sealing characteristics that prevent the migration ofmolten aluminum alloy beyond the defined surface boundary of the fillermaterial. This graphite tape is also resistant to heat and is chemicallyinert. GRAFOIL® graphite material is flexible, compatible, conformableand resilient. It can be made into a variety of shapes to fit anybarrier application. However, graphite barrier means may be employed asa slurry or paste or even as a paint film around and on the boundary ofthe filler material. GRAFOIL® is particularly preferred because it is inthe form of a flexible graphite sheet. In use, this paper-like graphiteis simply formed around the filler material.

As discussed earlier, the invention contemplates utilizing, for example,a GRAFOIL® sheet as both a barrier and a gating means. However, thegating means is distinct from the barrier means due to at least thepositioning of each relative to the matrix metal and preform.Specifically, the barrier can define ultimate movement of the moltenmatrix metal within the filler material or preform after infiltration,whereas the gating means controls the amount and/or location of contactof matrix metal with the filler material or preform both before andduring infiltration of the matrix metal. Moreover, it is possible forthe gating means to function as both a barrier and gating means. Forexample, after molten matrix metal passes through the gating means, themolten matrix metal may infiltrate the filler material or preform untilcontacting a back side of the gating means (e.g., infiltration couldoccur up to the point where the gating means actively contacts thefiller material or preform).

Other preferred barrier(s) for aluminum metal matrix alloys in nitrogenare the transition metal borides (e.g., titanium diboride (TiB₂)) whichare generally non-wettable by the molten aluminum metal alloy undercertain of the process conditions employed using this material. With abarrier of this type, the process temperature should not exceed about875° C., for otherwise the barrier material becomes less efficaciousand, in fact, with increased temperature infiltration into the barrierwill occur. The transition metal borides are typically in a particulateform (1-30 microns). The barrier materials may be applied as a slurry orpaste to the boundaries of the permeable mass of filler material whichpreferably is preshaped as a preform.

Other useful barriers for aluminum metal matrix alloys in nitrogeninclude low-volatile organic compounds applied as a film or layer ontothe external surface of the filler material. Upon firing in nitrogen,especially at the process conditions of this invention, the organiccompound decomposes leaving a carbon soot film. The organic compound maybe applied by conventional means such as painting, spraying, dipping,etc.

Moreover, finely ground particulate materials can function as a barrierso long as infiltration of the particulate material would occur at arate which is slower than the rate of infiltration of the fillermaterial.

Thus, the barrier means may be applied by any suitable means, such as bycovering the defined surface boundary with a layer of the barrier means.Such a layer of barrier means may be applied by painting, dipping, silkscreening, evaporating, or otherwise applying the barrier means inliquid, slurry, or paste form, or by sputtering a vaporizable barriermeans, or by simply depositing a layer of a solid particulate barriermeans, or by applying a solid thin sheet or film of barrier means ontothe defined surface boundary. With the barrier means in place,spontaneous infiltration substantially terminates when the infiltratingmatrix metal reaches the defined surface boundary and contacts thebarrier means.

Various demonstrations of the present invention are included in theExamples immediately following. However, these Examples should beconsidered as being illustrative and should not be construed as limitingthe scope of the invention as defined in the appended claims.

EXAMPLE 1

FIG. 1 shows an assembly, in cross section, which can be used to form ametal matrix composite body in accordance with the present invention.Specifically, a matrix metal (5) will spontaneously infiltrate a preform(2) through gating means (3). Particularly, a GRAFOIL® box (1) measuringabout 2×2×2 inches was assembled. The GRAFOIL® used to form the box (1)was a flexible graphite tape product that was obtained from UnionCarbide having a thickness of 0.015 inches. A preform (2) measuringabout 2×2×1/2 inches was placed into the box (1). The preform (2)comprised approximately 12 percent by volume chopped alumina fibers (atleast 90 percent by weight of the alumina fibers was Fiber FP producedby the Du Pont Company) and the alumina fibers were bound together withcolloidal alumina. The colloidal alumina/fiber weight ratio wasapproximately 1/4 and the balance of the preform volume comprisedinterconnected porosity. A gating means (3) was placed directly on topof the preform (2) in the GRAFOIL® box (1). The gating means (3)comprised another GRAFOIL® sheet which had five holes (30) (e.g.,passageways) punched therein. The gating means (3) was sealed along theseams (4) to the GRAFOIL® box (2) by applying a mixture of graphitepowder and colloidal silica. The aluminum alloy (5) was then placeddirectly on top of the gating means (3) in the box (1). The aluminumalloy (5) comprised about 10.5% Mg, 4% Zn, 0.5% silicon, 0.5% copper andremainder being aluminum. The aluminum alloy (5) was provided in twoingots, each measuring about 1×7/8×1/2 inches. The GRAFOIL® box (1)containing the ingots (5) and preform (2) was placed into a graphiteboat (6) which was partially filled with an alumina bedding (7) of 24grit, 38 ALUNDUM® obtained from Norton. The graphite boat (6) was thenfilled to a height approximately the same as the graphite box (1)contained therein. The primary purpose of the bedding (7) was to providesupport for the GRAFOIL® box (1).

The graphite boat (6) containing the assemblage of FIG. 1, was placedinto a controlled atmosphere electric resistance furnace (i.e., a vacuumfurnace which was pumped down to 1×10⁻⁴ torr). The furnace was thenbackfilled with nitrogen and heated up to about 200° C. in order topurge the environment in the furnace. During subsequent heating andinfiltration, nitrogen was passed through the vacuum furnace at a rateof about 2 liters per minute. The furnace was heated over a period ofabout 5 hours up to a temperature of about 700° C. The temperature wasmaintained for about 20 hours at which point the furnace was allowed tocool naturally to ambient temperatures.

After the furnace was cooled, the boat (6) and its contents were removedfrom the furnace. A carcass of aluminum alloy was readily removed fromthe preform with a hammer and chisel. Specifically, as shown in FIG. 3,the metal matrix composite (20) was substantially completely infiltratedby the matrix metal. The carcass of matrix metal (21) easily separatedfrom the formed metal matrix composite body (20). The circular regions(23) on each of the metal matrix composite body (20) and the carcass ofmatrix metal (21) correspond to the passageways (30) in the gating means(3). The areal contact between the matrix metal carcass (21) and metalmatrix composite (2e) was minimized, thus permitting easier separation.Moreover, the surface of the metal matrix (20) which was in contact withthe gating means (3) was sand blasted to remove remaining GRAFOIL®, thusresulting in a near-net shape metal matrix composite.

EXAMPLE 2

FIG. 2 shows an assembly, in cross section, which was used to form ametal matrix composite body in accordance with the present invention.Specifically, a GRAFOIL® box (8) measuring about 12×6×2 inches wasproduced. A preform (9) measuring about 12×6×0.3 inches was placed intothe box (8). The preform (9) was comprised of approximately 40.3 percentby volume of a continuous alumina fiber (at least 90 percent by weightof the alumina fibers was Fiber FP produced by the du Pont Company). Thealumina fiber was silica coated and bound together with 4 percent byvolume colloidal alumina wherein the fiber contained a 0°/90°orientation. A GRAFOIL® gating means (10) was placed directly above thepreform (9) and sealed to the GRAFOIL® box (8) in the manner discussedabove in Example 1. However, in this Example the gating means (10) hadonly a single rectangular opening (31) measuring about 5 inches×1 inch.An aluminum alloy ingot (11) weighing about 1700 grams and includingabout 10.5% by weight Mg was placed directly on top of the GRAFOIL®sheet gating means (10) in box (8). The alloy (11) was positioned insuch a manner that when the aluminum alloy became molten it would flowspontaneously through the gating means (10) and into the preform (9).Additionally, two stainless steel bars (12) were placed at each end ofthe GRAFOIL® gating means (10), but did not contact with the aluminumalloy (11). The bars (12) served to hold the gating means (10) inposition during spontaneous infiltration. The GRAFOIL® box (8) was thenplaced into a graphite boat (14). A bedding (13) of 24 grit ALUNDUM® wasplaced around the box (8) in the manner described in accordance withExample 1.

The graphite boat (14) was then placed into a vacuum furnace and purgedas discussed above in Example 1. During the subsequent heating andinfiltration steps, nitrogen was passed through the vacuum furnace at arate of about 2.5 liters per minute. The furnace was heated to about725° C. over a period of about 5 hours. This temperature was maintainedfor about 45 hours, after which the furnace was turned off and allowedto cool naturally. The graphite boat was removed from the furnace andthe carcass alloy removed from the preform as discussed above inExample 1. Specifically, as shown in FIG. 4, the metal matrix composite(40) was substantially completely infiltrated by the matrix metal. Thecarcass of matrix metal (41) easily separated from the formed metalmatrix composite (40) by pulling the two bodies apart. The rectangularregion (42) on both bodies corresponds to the passageway (31) in thegating means (10) which permitted molten matrix metal to flowtherethrough.

In each of these Examples, the GRAFOIL® box and gating means werereadily removed, when necessary, by light sand or grit blasting.However, in some instances it may be necessary to lightly grind, etch,etc., to remove residual processing materials.

These two Examples demonstrate two advantages of the invention.Particularly, after the spontaneous infiltration of the metal matrixalloy into a preform has occurred, the carcass of matrix metal will nothave to be machined to separate it from the metal matrix composite body.Further, the gating means prevented warping of the metal matrixcomposite, upon cooling. Specifically, the aluminum in the matrix metalcarcass has a higher coefficient of thermal expansion than the formedmetal matrix composite. Accordingly, as the carcass cools it shrinks ata higher rate than the infiltrated composite, and if the carcass is indirect contact with the formed metal matrix composite the carcass willtend to cause the composite to bend or warp (e.g., become U-shaped). Thegating means of the invention provides a solution to reduce theundesirable aspects of each of the problems discussed above.

While the preceding Examples have been described with particularity,various modifications to these Examples may occur to an artisan ofordinary skill, and all such modifications should be considered to bewithin the scope of the claims appended hereto.

EXAMPLE 3

FIG. 5 shows, in cross-section, a set-up employing a gating means toform a metal matrix composite body in accordance with the presentinvention. Specifically, a graphite boat 50 measuring about 8 inches(203 mm) by about 4 inches (102 mm) by about 3 inches (76 mm) high wascoated on its interior surfaces with a colloidal graphite slurry 52(DAG® 154, Acheson Colloids Company, Port Huron, Mich.) which wasallowed to dry in air at room temperature for about four (4) hours. Auniform layer of magnesium powder 56 (-100 mesh, Hart Corporation,Tamaqua, Pa.) was adhered to the bottom of the graphite boat 50 using anadhesive material 54 comprising by volume about 40% RIGIDLOCK® graphitecement (Polycarbon Corporation, Valencia, Calif.) and the balance ethylalcohol.

About 1000 grams of a filler material comprising by weight about 4percent magnesium powder (-325 mesh, Hart Corporation) and the balance220 grit 39 CRYSTOLON® green silicon carbide (Norton Company, Worcester,Mass.) were prepared by placing the magnesium and silicon carbideparticulates into a 2 liter plastic jar and roll mixing for about 2hours. About 704 grams of the roll mixed filler material 58 were thenpoured into the graphite boat 50 and leveled. A layer of -100 meshmagnesium powder 60 was then placed on top of the leveled fillermaterial. A sheet of approximately 14 mil (0.36 mm) thick GRAFOIL®graphite foil 62 (Union Carbide Corporation, Danbury, Conn.)substantially the same length and width as the internal dimensions ofthe boat, and containing an approximately 11/2 inch (38 mm) diameterhole centered in the sheet, was placed into the graphite boat 50 on topof the layer of magnesium powder 60. A graphite ring 64 measuring about13/4 inch (44 mm) in diameter and about 3/8 inch (10 mm) in height wascentered over the hole in the graphite foil and glued to the graphitefoil using colloidal graphite (DAG® 154). The ring cavity 66 was filledwith magnesium powder (-100 mesh, Hart Corporation). A 90 grit 38ALUNDUM® alumina particulate material 68 (Norton Company, Worcester,Mass.) was then placed into the graphite boat 50 around the outside ofthe graphite ring 64 to a level substantially flush with the top surfaceof the graphite ring 64. A matrix metal ingot 70 comprising by weightabout 12 percent silicon, 2 percent magnesium and the balance aluminum,and weighing about 1170 grams, was placed into the graphite boat 50 andcentered over the graphite ring 64.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at room temperature. The furnace chamberwas evacuated to a vacuum of about 30 inches (762 mm) of mercury, thenbackfilled with nitrogen gas to establish a gas flow rate of about 3liters per minute within the furnace. The furnace temperature wasincreased to about 525° C. at a rate of about 200° C. per hour andmaintained at a temperature of about 525° C. for about an hour. Thetemperature was then increased to about 775° C. at a rate of about 200°C. per hour, maintained at a temperature of about 775° C. for about 10hours, then decreased to about 675° C. at a rate of about 200° C. perhour. At a temperature of about 675° C., the graphite boat and itscontents were removed from the furnace and placed on a water cooledaluminum quench plate. A FEEDOL® 9 particulate hot topping material(Foseco, Inc., Cleveland, Ohio) was poured onto the top of the residualmolten matrix metal. A CERABLANKET® ceramic fiber blanket (ManvilleRefractory Products, Denver, Colo.) measuring about 2 inches (51 mm)thick was placed over the top of the lay-up. After cooling tosubstantially room temperature, the lay-up was removed from the graphiteboat and the uninfiltrated alumina powder was poured out. Using moderatehand pressure, the carcass of residual matrix metal was then easilyseparated from the formed metal matrix composite at the boundary betweenthe metal matrix composite and the unreinforced matrix metal inside thegraphite ring of the gating means.

EXAMPLE 4

FIG. 6 shows, in cross-section, a set-up incorporating a gating means toform a metal matrix composite body in accordance with the presentinvention. Specifically, a particulate mixture comprising about 200grams of Grade A-200 aluminum nitride (Advanced Refractory Technologies,Inc., Buffalo, N.Y.) and about 10 grams of magnesium (-325 mesh, HartCorporation, Tamaqua, Pa.) was placed into a one liter plastic jar androll mixed for about 2 hours. The mixture was then placed into apressing die and pressed under a uniaxially applied pressure of about5000 psi (35 MPa) to form a preform 80 measuring about 3 inches (76 mm)by about 3 inches (76 mm) by about 0.75 inch (19 mm) thick. A graphitering 82 measuring about 1.75 inches (44 mm) in diameter and about 0.375inch (10 mm) high was centered on one of the 3 inch square (76 mm) facesof the preform 80. The surfaces of the preform 80 exterior to the gatingmeans 82 were aerosol spray coated with AERODAG® GS colloidal graphite84 (Acheson Colloids Company, Port Huron, Mich.). The graphite ring 82and the coated preform 80 were then allowed to dry in air at roomtemperature for about 3 to 5 hours.

A graphite boat 86 measuring about 6 inches (152 mm) square and about 3inches (76 mm) high was filled to a depth of about 1/2 inch (13 mm) with220 grit 38 ALUNDUM® particulate alumina 88 (Norton Company). Thegraphite ring 82 and coated preform 80 were then placed into thegraphite boat 86 on top of the alumina 88. Additional alumina was placedinto the graphite boat 86 around the preform 80 and the graphite ring 82up to a level substantially flush with the top of the graphite ring 82,but somewhat higher out near the walls of the graphite boat 86 toestablish the gating means. The interior space 90 within the graphitering 82 was then filled with -50 mesh magnesium powder (HartCorporation). An approximately 623 gram ingot of matrix metal 92comprising commercially pure aluminum metal and measuring about 3 inches(76 mm) square and about 11/2 inches (38 mm) thick was then placed intothe graphite boat 86 and centered over the graphite ring 82.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at room temperature. The furnace chamberwas evacuated to a vacuum of about 30 inches (762 mm) of mercury andthen backfilled with nitrogen gas to establish a gas flow rate of about3 liters per minute within the furnace. The furnace temperature wasincreased to about 525° C. at a rate of about 200° C. per hour,maintained at a temperature of about 525° C. for about an hour, thenincreased to about 775° C. at a rate of about 200° C. per hour. Aftermaintaining a temperature of about 775° C. for about 7 hours, thetemperature was decreased to about 675° C. at a rate of about 200° C.per hour. At a temperature of about 675° C., the graphite boat and itscontents were removed from the furnace and placed onto a water cooledaluminum quench plate. A FEEDOL® 9 particulate hot topping material(Foseco, Inc., Cleveland, Ohio) was poured onto the top of the residualmolten matrix metal. A CERABLANKET® ceramic fiber blanket (ManvilleRefractory Products, Denver, Conn.) measuring about 2 inches (51 mm)thick was placed on top of the lay-up. After cooling to substantiallyroom temperature, the lay-up was removed from the graphite boat, and theuninfiltrated alumina powder was poured out. The carcass of residualmatrix metal was then separated from the formed metal matrix compositeby cutting through the graphite ring and the unreinforced matrix metalwithin.

EXAMPLE 5

In this Example, four metal matrix composite bodies were formed in amanner substantially identical to the method of Example 4, except thatthe four preform bodies were placed into a single graphite boat andprocessed simultaneously. Specifically, about 10,000 grams of densealumina milling media, each particle measuring about 0.94 inch (24 mm)in diameter, were placed into an 8 liter porcelain ball mill. About 5000grams of -325 mesh T64 Tabular Alumina particulate alumina (AlcoaIndustrial Chemicals Div., Bauxite, Ark.) filler material was added tothe mill, and the alumina filler was dry ball milled for about sixhours. The milling media was then removed from the ball mill, and about100 grams of -325 mesh magnesium (Hart Corporation, Tamaqua, Pa.) wereadded to the alumina filler in the ball mill, which was then roll mixedfor about 2 hours. The roll mixed admixture was then transferred to themixing chamber of a high intensity mixer (Eirich Intensive Mixer, ModelRV 02, Eirich Machines Inc., Uniontown, Pa.), and about 570 grams of abinder solution comprising by weight about 20 percent QPAC™polypropylene carbonate binder (Air Products and Chemicals, Inc.,Emmaus, Pa.), 10 percent propylene carbonate (Fisher Scientific,Pittsburgh, Pa.) and the balance acetone were uniformly dispersed intothe powder with the high intensity mixer. The powder was then uniaxiallypressed under an applied pressure of about 10,000 psi (69 MPa) into apreform measuring about 3 inches (76 mm) square and about 5/8 inch (16mm) thick. Three other such preforms were pressed in a similar manner.

A graphite ring measuring about 2 inches (51 mm) in diameter and about3/8 inch (10 mm) high was glued to the center of one of the 3 inch (76mm) square faces on each of the four preforms using a DAG® 154 colloidalgraphite paste (Acheson Colloid Company, Port Huron, Mich.). The fourpreforms were allowed to dry in air at room temperature for about 3 to 5hours. After drying, the four preforms were placed into a GRAFOIL®graphite foil (Union Carbide Corp., Danbury, Conn.) box measuring about8 1/2 inches (216 mm) by 111/2 (292 mm) inches by about 4 inches (102mm) high and spaced between 1/2 inch (13 mm) and 1 inch (25 mm) apart. Abedding material comprising by weight about 15 percent P54 borosilicateglass frit (Mobay Chemical Corporation, Inorganic Chemicals Div.,Baltimore, Md.) and the balance equal weight proportions of 90, 220, and500 grit E1 ALUNDUM® alumina (Norton Company, Worcester, Mass.) wasplaced into the graphite foil box around the preforms and up to a levelsubstantially flush with the top of the graphite rings, but slightlyhigher toward the walls of the graphite foil box. The interior of eachgraphite ring was filled with -100 mesh magnesium powder (HartCorporation). Two matrix metal ingots comprising by weight about 9.5 to10.6 percent magnesium, ≦0.25% silicon, ≦0.3% iron, ≦0.25% copper,≦0.15% manganese, ≦0.15% zinc, ≦0.25% titanium, and the balancealuminum, each ingot measuring about 8 inches (203 mm) by about 4 inches(102 mm) by about 11/2 inches (38 mm) and weighing about 2160 grams,were placed into the graphite foil box such that each matrix metal ingotcontacted two preform/graphite tube assemblies. The graphite foil boxand its contents were then placed into a graphite boat measuring about 9inches (229 mm) by about 12 inches (305 mm) by about 4 inches (102 mm)tall.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at room temperature. The furnace chamberwas evacuated to a vacuum of about 30 inches (762 mm) of mercury, thenbackfilled with nitrogen gas to establish a gas flow rate of about 4liters per minute within the furnace. The furnace temperature wasincreased to about 250° C. in one hour, held at about 250° C. for aboutone hour, then increased to about 450° C. at a rate of about 50° C. perhour. After maintaining a temperature of about 450° C. for about 1 hour,the temperature was then increased to about 775° C. at a rate of about150° C. per hour, held at about 775° C. for about 7 hours, thendecreased to about 675° C. at a rate of about 200° C. per hour. At atemperature of about 675° C., the graphite boat and its contents wereremoved from the furnace and placed onto a water cooled aluminum quenchplate. A FEEDOL@ 9 particulate hot topping material (Foseco Inc.,Cleveland, Ohio) was poured onto the top of the residual molten matrixmetal. A CERABLANKET® ceramic fiber blanket (Manville RefractoryProducts, Denver, Colo.) measuring about 2 inches (51 mm) thick wasplaced on top of the lay-up in the graphite boat. After cooling tosubstantially room temperature, the lay-up was removed from the graphiteboat. The bedding of alumina and glass frit was removed from the lay-upwith light hammer blows to reveal that matrix metal had infiltrated thepreforms to produce four metal matrix composites. The formed metalmatrix composites were then removed from the carcass of residual matrixmetal by cutting through the graphite rings and the unreinforced matrixmetal contained within using a saw.

EXAMPLE 6

A metal matrix composite body was formed in a manner substantiallyidentical to the procedure for Example 4, as shown in FIG. 6, exceptthat a graphite foil was positioned between the preform and the loosealumina bedding material, rather than a coating of colloidal graphite.Specifically, an aqueous solution of BLUONIC® A colloidal alumina(Buntrock Industries, Inc., Lively, Va.) weighing about 261.4 grams wasdiluted with about 522.8 grams of water and placed into a 2 literplastic jar. About 1280.9 grams of 220 grit 39 CRYSTOLON® green siliconcarbide particulate (Norton Co., Worcester, Mass.) and about 548.9 gramsof 500 grit 39 CRYSTOLON® green silicon carbide particulate were addedto the jar to prepare a slurry for sediment casting. The slurry was rollmixed for about 45 minutes, then poured into a silicone rubber mold withan internal cavity measuring about 7 inches (178 mm) square and about11/2 inches (38 mm) deep. The mold was vibrated on a vibration table forabout 2 hours to assist in sedimentation, and any excess water on thesurface of the formed sediment cast preform was removed with a papertowel. The silicone rubber mold was then removed from the vibrationtable and placed into a freezer. After the residual water in the preformhad thoroughly frozen, the silicone rubber mold and preform were removedfrom the freezer, and the frozen sediment cast preform was withdrawnfrom the mold. The preform was placed on a bed of 90 grit 38 ALUNDUM®alumina particulate material (Norton Company) and allowed to dry in airat room temperature for about 16 hours.

After drying, the sediment cast preform was transferred to a bedding of90 grit alumina supported by a refractory plate and placed into aresistance heated air atmosphere furnace for firing. The furnacetemperature was increased from substantially room temperature to atemperature of about 1050° C. in a period of about 10 hours. Aftermaintaining a temperature of about 1050° C. for about 2 hours, thetemperature was decreased to substantially room temperature in a periodof about 10 hours.

A GRAFOIL® graphite foil (Union Carbide Company, Danbury, Conn.) boxmeasuring about 8.5 inches (216 mm) square and about 4 inches (102 mm)high was placed into a graphite boat having interior dimensions of about9 inches (229 mm) square and about 4 inches (102 mm) high, and the firedsediment cast preform was placed into the bottom of the graphite foilbox. A bedding material comprising by weight about 15% F-69 borosilicateglass frit (Fusion Ceramics, Inc., Carrollton, Ohio) and the balanceequal proportions of 90, 220, and 500 grit E1 ALUNDUM® alumina wasplaced into the graphite foil box around the fired sediment cast preformto a level substantially flush with the top of the preform. A thinsurface layer of -100 mesh magnesium powder (Hart Corporation, Tamaqua,Pa.) was sprinkled over the top of the preform.

A graphite ring with an inside diameter measuring about 21/2 inches (64mm) and a height of about 1/2 inch (13 mm) was centered over anapproximately 21/2 inch (64 mm) diameter hole in an about 7 inch (178mm) square sheet of 14 mil (0.36 mm) thick graphite foil. The graphitering was then adhered to the graphite foil with a thin layer of anadhesive comprising by volume about 40% RIGIDLOCK® graphite cement(Polycarbon Corporation, Valencia, Calif.) and the balance ethylalcohol. The joined graphite components were allowed to dry in air atroom temperature for about four (4) hours.

The graphite foil and ring assembly was then placed into the graphitefoil box on top of the layer of -100 mesh magnesium powder with thegraphite ring facing up. The inside of the graphite ring was then filledwith a dry particulate mixture comprising by weight about 1% -100 meshmagnesium powder, 1% -325 mesh magnesium powder, 29% 90 grit 39CRYSTOLON® green silicon carbide and the balance 54 grit green siliconcarbide. Additional bedding material (particulate mixture of alumina andglass frit) was then poured into the graphite box around the graphitefoil and ring assembly to a height substantially flush with the top ofthe graphite ring, but somewhat higher out toward the walls of thegraphite box. An approximately 1736 gram ingot of matrix metal measuringabout 5 inches (127 mm) square and about 11/2 inches (38 mm) thick andcomprising by weight about 12% silicon, 6% magnesium, and the balancealuminum, was placed into the graphite foil box and centered over thegraphite ring.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at room temperature. The furnace chamberwas evacuated to a vacuum of about 30 inches (762 mm) of mercury andthen backfilled with nitrogen gas to establish a gas flow rate of about3 liters per minute within the furnace. The furnace temperature wasincreased to about 825° C. at a rate of about 150° C. per hour, held atabout 825° C. for about 20 hours, then decreased to about 700° C. at arate of about 200° C. per hour. At a temperature of about 700° C., thegraphite boat and its contents were removed from the furnace and placedonto a water cooled aluminum quench plate. A FEEDOL® 9 particulate hottopping material (Foseco, Inc., Cleveland, Ohio) was poured onto the topof the residual molten matrix metal. An approximately 2 inch (51 mm)thick layer of CERABLANKET® ceramic fiber insulation (ManvilleRefractory Products, Denver, Colo.) was placed on top of the graphiteboat to further assist in directional solidification. After cooling tosubstantially room temperature, the lay-up was removed from the graphiteboat.

The bedding of alumina and glass frit material was removed from aroundthe lay-up with light hammer blows to reveal that matrix metal hadinfiltrated the sediment cast preform to produce a metal matrixcomposite of substantially the same size and shape as the preform. Thecarcass of residual matrix metal was then easily broken off from theformed metal matrix composite through the use of moderate hand pressure.The two pieces separated at the boundary between the metal matrixcomposite material inside of the graphite ring and the carcass ofresidual matrix metal above. The graphite ring and the metal matrixcomposite material within the ring were separated from the metal matrixcomposite by diamond machining.

EXAMPLE 7

This example further demonstrates a method for fabricating a metalmatrix composite body using a gating means in accordance with thepresent invention.

A tape cast silicon carbide preform, obtained from Keramos Industries,Inc., Morrisville, Pa., having dimensions of about 8 inches (203 mm) byabout 7 inches (177 mm) by about 0.145 inch (4 mm) thick and comprisingby weight about 70% 220 grit, 10% 500 grit, 10% 800 grit, and 10% 1000grit 39 CRYSTOLON® green silicon carbide particulate (Norton Company,Worcester, Mass.) was placed, with its flatest side facing down, on aperforated cordierite plate. The preform was covered with a sheet ofFIBERFRAX® 907-J fiber insulation paper (The Carborundum Company,Niagara Falls, N.Y.). A second cordierite plate was placed on top of thefiber insulation paper to form an assembly. The assembly was placedwithin a room temperature air atmosphere furnace. The temperature in thefurnace was increased from about room temperature to about 425° C. at arate of about 50° C. per hour. After reaching a temperature of about425° C., the temperature in the furnace was increased to about 1050° C.at a rate of about 200° C. per hour. After maintaining a temperature ofabout 1050° C. for about 1 hour, the temperature in the furnace wasdecreased to about room temperature in about 5 hours. The assembly wasremoved from the furnace, and the weight of the preform was determinedto be about 257.04 grams.

The preform was placed on a rotatable platform with its flat side facingup. A first edge and the flat side of the preform were spray coated withKRYLON® acrylic spray coating (Borden, Inc., Columbus, Ohio). Therotatable platform and preform were rotated 90° and a second edge of thepreform and the flat side were spray coated with KRYLON® acrylic spraycoating. This procedure was repeated until all four edges of the preformwere spray coated with one coat of KRYLON® acrylic spray coating and theflat side of the preform was coated with four coats of KRYLON® acrylicspray coating. A temperature of about 65° C. was established within anair atmosphere furnace, and the preform was transferred from therotatable platform to the air atmosphere furnace. After about 10minutes, the preform was removed from the air atmosphere furnace andplaced under a fume hood until the coating had substantially dried. Thepreform was removed from the fume hood, placed on a balance, and apreform weight of about 257.22 grams was recorded.

A mixture comprising by volume about 50% DAG® 154 colloidal graphite(Acheson Colloids, Port Huron, Mich.) and about 50% denatured ethanolwas prepared. The preform was placed on a rotatable platform with itsflat side facing up, and an air brush was used to apply a thin layer ofthe mixture to a first edge and the flat side of the preform. Theplatform and preform were rotated 90° and the mixture was applied to asecond edge and the flat side of the preform. This procedure wasrepeated until all four edges of the preform were coated with two coatsand the flat side of the preform was coated with eight coats of themixture, although due to overspray and run off of the mixture during thecoating of the flat side of the preform, the thickness of the coatingson the edges approximately equalled the thickness of the coatings on theflat side of the preform, thus providing a barrier layer to the preform.Then the preform was allowed to dry. After the preform was substantiallydry, the preform was placed on a balance and a preform weight of about257.92 grams was recorded. The preform was placed on a rotatableplatform with the flat side facing down. The preform was spray coatedwith KRYLON® acrylic spray coating in a manner substantially the same asfor the KRYLON® coating described above. A temperature of about 65° C.was established within an air atmosphere furnace, then the preform wastransferred from the rotatable platform to the air atmosphere furnaceand heated for about 10 minutes. The preform was removed from the airatmosphere furnace and placed under a fume hood. After the preform hadsubstantially dried, the preform was placed on a balance and a preformweight of about 258.27 grams was recorded. The preform was then placed,with its flat side facing down, on a rotatable platform and an air brushwas used to apply a thin layer of the mixture to the top portion of thepreform. The mixture was then allowed to dry completely. A total ofthree coats of the mixture were applied in this manner to form a gatingmeans, then the preform was placed on a balance and a preform weight ofabout 259.23 grams was recorded.

As shown in FIG. 7, a sheet of GRAFOIL® graphite foil (71) (UnionCarbide Company, Danbury, Conn.) measuring about 131/4 inches (337 mm)by about 91/4 inches (235 mm) by about 0.015 inch (0.4 mm) thick wasplaced into the bottom of a graphite boat (72) having inner dimensionsof about 131/4 inches (337 mm) by about 91/4 (235 mm) inches by about 1inch (25 mm) high. An about 3/8 inch (9 mm) thick layer of beddingmaterial (73), comprising by weight about 10% F-69 borosilicate glassfrit (Fusion Ceramics, Inc., Carrollton, Ohio) and the balancecomprising about 70% by weight 36 grit and about 30% by weight 60 gritE-38 ALUNDUM® alumina (Norton Company, Worcester, Mass.) was poured intothe graphite boat (72) on top of the GRAFOIL® sheet (71). A foam brushwas used to establish a level layer of bedding material (73).

A matrix metal ingot (74) weighing about 1462.92 grams and comprising byweight about 20% silicon, 5% magnesium and the balance aluminum, wasplaced into an ethanol bath. The surface of the matrix metal ingot (74)was cleaned by hand utilizing a paper towel, then the matrix metal ingot(74) was removed from the ethanol bath and placed within an airatmosphere furnace. A temperature of about 68° C. was established withinthe furnace, and after heating the matrix metal ingot for about 15minutes, the matrix metal ingot (74) was removed from the furnace andplaced on top of the bedding material (73) within the graphite boat(72). Additional bedding material (73) was poured into the graphite boat(72) around the matrix metal ingot (74) to a level substantially thesame as the top portion of the matrix metal ingot (74). A sheet ofGRAFOIL® graphite foil (Union Carbide Company, Danbury, Conn.) measuringabout 81/4 inches (210 mm) by about 71/4 inches (184 mm) by about 0.005inches (0.1 mm) thick was prepared by first cutting a rectangular holemeasuring about 77/8 inches (200 mm) by about 67/8 inches (175 mm) inthe center of the GRAFOIL® sheet to produce a GRAFOIL® frame (75). Oneside of the GRAFOIL® frame (75) was spray coated with KRYLON® acrylicspray coating. The GRAFOIL® frame (75) was then centered on top of thematrix metal ingot (74) with the acrylic coating in contact with thematrix metal ingot (74). The portion of the matrix metal ingot (74)within the inner boundaries of the GRAFOIL® frame (75) was spray coatedwith KRYLON® acrylic spray coating. About 5.6 grams of -50 mesh atomizedmagnesium (76) (Hart Corporation, Tamaqua, Pa.) were sprinkled onto theportion of the matrix metal ingot (74) within the inner boundaries ofthe GRAFOIL® frame (75). The GRAFOIL® frame (75), the -50 mesh atomizedmagnesium (76) and the matrix metal ingot (74) were then spray coatedwith KRYLON® acrylic spray coating, and the acrylic spray coating wasallowed to dry for about 3 minutes. The preform (77) was centered on topof the GRAFOIL® frame (75), with the flat side of the preform (77) incontact with the GRAFOIL® frame (75), the -50 mesh atomized magnesium(76) and the matrix metal ingot (74).

The graphite boat (72) and its contents were placed into a resistanceheated controlled atmosphere furnace at about room temperature. Thefurnace was sealed, evacuated to about 30 inches (762 mm) of mercuryvacuum, and backfilled with nitrogen gas to about atmospheric pressure.A nitrogen gas flow rate of about 5 liters per minute was establishedwithin the furnace. The temperature in the furnace was increased fromabout room temperature to about 225° C. at a rate of about 200° C. perhour. After maintaining a temperature of about 225° C. for about 52hours, the temperature in the furnace was increased to about 850° C. ata rate of about 200° C. per hour. After maintaining a temperature ofabout 850° C. for about 10 hours, the temperature in the furnace wasdecreased to about 825° C. at a rate of about 200° C. per hour. Thegraphite boat (72) and its contents were then removed from the furnace.An about 15 inch (381 mm) by about 11 inch (279 mm) by about 2 inch (51mm) thick layer of CERABLANKET® ceramic insulation material (ManvilleRefractory Products, Denver, Colo.) was placed onto a graphite table. Asingle sheet of GRAFOIL® graphite foil having dimensions of about 15inches (381 mm) by about 11 inches (279 mm) by about 0.015 inch (0.38mm) thick was placed on top of the CERABLANKET® fiber insulationmaterial. The graphite boat (72) and its contents were placed on top ofthe GRAFOIL® graphite foil and allowed to cool. After about 13 minutes,light chisel blows were applied to the solidified matrix metal causingthe formed metal matrix composite to separate from the matrix metal.

EXAMPLE 8

This Example further demonstrates a method for fabricating metal matrixcomposite bodies using a gating means in accordance with the presentinvention.

A total of 18 injection molded preforms comprising essentially 220 gritand 1000 grit silicon carbide particulate were obtained from TechnicalCeramics Laboratories, Inc., Alpharetta, Ga.

An about 12 inch (305 mm) long by about 6 inch (152 mm) wide sheet ofFIBERFRAX® 907-J fiber insulation paper (The Carborundum Company,Niagara Falls, N.Y.) was placed onto a refractory plate havingdimensions of about 12 inches (304 mm) long by about 6 inches (152 mm)wide by about 1 inch (25 mm) thick and the preforms were placed onto thefiber insulation paper. The refractory plate/fiber insulationpaper/preform assembly was placed within a room temperature airatmosphere furnace. The temperature in the furnace was increased fromabout room temperature to about 425° C. in about 8 hours. Thetemperature in the furnace was then increased to about 1050° C. in about3 hours. After maintaining a temperature of about 1050° C. for about 1hour, the temperature in the furnace was decreased to about roomtemperature in about 5 hours. The refractory plate/fiber insulationpaper/preform assembly was removed from the furnace.

Each preform was weighed and the weight recorded, with preform weightsranging from about 5.78 grams to about 5.96 grams. All 18 preforms werethen treated in the following manner. A preform was placed on arotatable platform with its flat side facing up. The flat side and edgesof the preform were lightly spray coated with KRYLON® acrylic spraycoating (Borden, Inc., Columbus, Ohio). The rotatable platform andpreform were slowly rotated to insure that each edge of the preform waslightly coated with KRYLON® acrylic spray coating. A temperature ofabout 65° C. was established within an air atmosphere furnace, and thepreform was transferred from the rotatable platform to the airatmosphere furnace. After about 10 minutes, the preform was removed fromthe air atmosphere furnace and placed under a fume hood until thecoating had substantially dried. A mixture comprising by volume about50% DAG® 154 colloidal graphite (Acheson Colloids, Port Huron, Mich.)and about 50% denatured ethanol was prepared. The preform was placed ona rotatable platform with its flat side facing up, and an air brush wasused to apply a thin layer of the mixture to the flat side and edges ofthe preform. The platform and preform were rotated to insure that alledges were spray coated with a substantially uniform layer of mixture.The mixture was allowed to dry and the procedure was repeated. A totalof about 0.04 grams of the mixture was applied to form a barrier on thetop and edges of the preform. The preform was placed on a rotatableplatform with its flat side facing down. The preform was spray coatedwith KRYLON® acrylic spray coating in a manner substantially the same asfor the KRYLON® coating described above. A temperature of about 65° C.was established within an air atmosphere furnace, the preform wastransferred from the rotatable platform to the air atmosphere furnaceand heated for about 10 minutes. The preform was removed from the airatmosphere furnace and placed under a fume hood. After the preform hadsubstantially dried, the preform was placed, with its flat side facingdown, on a rotatable platform and an air brush was used to apply a thinlayer of the mixture to the exposed surfaces of the preform. The mixturewas then allowed to dry completely. A total of 3 layers of the mixturewere applied in this manner to form a gating means, with a total weightof about 0.02 grams of the mixture being applied.

As shown in FIG. 8, a sheet of GRAFOIL® graphite foil (101) (UnionCarbide Company, Danbury, Conn.), measuring about 131/4 inches (337 mm)by about 91/4 inches (235 mm) by about 0.015 inch (0.4 mm) thick wasplaced into the bottom of the graphite boat (100) having innerdimensions of about 131/4 inches (337 mm) by about 91/4 inches (235 mm)by about 1 inch (25 mm) high. An about 3/8 inch (9 mm) thick layer ofbedding material (102), comprising by weight about 60% 36 grit E-38ALUNDUM® alumina (Norton Company, Worcester, Mass.), about 17% 90 gritE-1 ALUNDUM® alumina (Norton Company) and about 13% F-69 borosilicateglass frit (Fusion Ceramics, Inc., Carrollton, Ohio) was poured into thegraphite boat (100) on top of the GRAFOIL® sheet (101). A foam brush wasused to establish a level layer of bedding material (102).

A matrix metal ingot (103) weighing about 1013 grams and comprising byweight about 15% silicon, 5% magnesium and the balance aluminum, wasplaced into an ethanol bath. The surface of the matrix metal ingot (103)was cleaned by hand utilizing a paper towel, then the matrix metal ingot(103) was removed from the ethanol bath and placed within an airatmosphere furnace. A temperature of about 68° C. was established withinthe furnace, and after heating the matrix metal ingot for about 15minutes, the matrix metal ingot (103) was removed from the furnace andplaced on top of the bedding material (102) within the graphite boat(100). Additional bedding material (102) was poured into the graphiteboat (100) around the matrix metal ingot (103) to a level substantiallythe same as the top portion of the matrix metal ingot (103). A sheet ofGRAFOIL® graphite foil (Union Carbide Company, Danbury, Conn.) having alength and width substantially the same as that of the exposed surfaceof the matrix metal ingot (103) and a thickness of about 0.005 inches(0.1 mm) was prepared by first cutting out 18 rectangular holesmeasuring about 1/8 inch (3 mm) less than the length of the preforms andabout 1/8 inch (3 mm) less then the width of the preforms. One side ofthe GRAFOIL® graphite foil (104) was spray coated with KRYLON® acrylicspray coating. The GRAFOIL® graphite foil (104) was then centered on topof the matrix metal ingot (103) with the acrylic coating in contact withthe matrix metal ingot (103). The portions of the matrix metal ingotwithin the inner boundaries of the holes in the GRAFOIL® graphite foil(104) were spray coated with KRYLON® acrylic spray coating. About 0.13gram of -50 mesh atomized magnesium (105) (Hart Corp., Tamaqua, Pa.) wassprinkled onto the portions of the matrix metal ingot (103) within theinner boundaries of each hole in the GRAFOIL® graphite foil (104). TheGRAFOIL® graphite foil (104), the -50 mesh atomized magnesium (105) andthe matrix metal ingot (103) were then spray coated with KRYLON® acrylicspray coating, and the acrylic spray coating was allowed to dry forabout 3 minutes. One preform (106) was centered on top of each cut-outportion of the GRAFOIL® graphite foil (104), with the flat side of eachpreform (106) in contact with the GRAFOIL® graphite foil (104), the -50mesh atomized magnesium (105) and the matrix metal ingot (103).

The graphite boat (100) and its contents were placed into a resistanceheated controlled atmosphere furnace at about room temperature. Thefurnace was sealed, evacuated to about 30 inches (762 mm) of mercuryvacuum, and backfilled with nitrogen gas to about atmospheric pressure.A nitrogen gas flow rate of about 5 liters per minute was establishedwithin the furnace. The temperature in the furnace was increased fromabout room temperature to about 225° C. in about 1 hour. Aftermaintaining a temperature of about 225° C. for about 2 hours, thetemperature in the furnace was increased to about 850° C. at a rate ofabout 200° C. per hour. After maintaining a temperature of about 850° C.for about 10 hours, the temperature in the furnace was decreased toabout 825° C. at a rate of about 200° C. per hour. The graphite boat(100) and its contents were then removed from the furnace. An about 15inch (381 mm) by about 11 inch (279 mm) by about 2 inch (51 mm) thicklayer of CERABLANKET® ceramic insulation material (Manville RefractoryProducts, Denver, Colo.) was placed onto a graphite table. A singlesheet of GRAFOIL® graphite foil having dimensions of about 15 inches(381 mm) by about 11 inches (279 mm) by about 0.015 inch (0.38 mm) thickwas placed on top of the CERABLANKET® fiber insulation material. Thegraphite boat (100) and its contents were placed on top of the GRAFOIL®graphite foil and allowed to cool. After the graphite boat (100) and itscontents had reached substantially room temperature, the formed metalmatrix composite bodies were easily removed by hand from the surface ofthe solidified matrix metal.

EXAMPLE 9

This Example provides comparative examples which demonstrate thebenefits of fabricating a metal matrix composite body using a gatingmeans in accordance with the present invention. Sample A illustrates amethod for forming a metal matrix composite body utilizing a gatingmeans of the present invention. Sample B illustrates a method of forminga comparable metal matrix composite body without a gating means.

Sample A

A metal matrix composite body was fabricated in essentially the samemanner as described in Example 7, with the following exceptions. Thepreform dimensions were about 61/2 inches (165 mm) by about 7 inches(177 mm) by about 0.08 inch (2 mm) thick. When removed from the airatmosphere furnace, the preform weight was determined to be about 115.79grams. After the first application of KRYLON® acrylic spray coating, apreform weight of about 116.54 grams was recorded. After the first DAG®154 colloidal graphite mixture was applied, the preform weight wasdetermined to be about 117.02 grams. After the second coating of KRYLON®acrylic spray coating, the preform weight was determined to be about117.39 grams. After the final application of the mixture, the preformweight was determined to be about 118.14 grams. The bedding materialcomprised by weight about 13% F-69 borosilicate glass frit (FusionCeramics, Inc., Carrollton, Ohio), about 60% 36 grit E38 ALUNDUM®alumina (Norton Company, Worcester, Mass.) and about 17% 90 grit E1ALUNDUM® alumina (Norton Company). The matrix metal ingot had a weightof about 1404.28 grams. The GRAFOIL® graphite foil (Union CarbideCompany, Danbury, Conn.) window had outer dimensions of about 63/4inches (171 mm) by about 71/4 inches (184 mm). The rectangular holes inthe center of the GRAFOIL® sheet had dimensions of about 63/8 inches(162 mm) by about 67/8 inches (175 mm). About 4.6 grams of -50 meshatomized magnesium was sprinkled onto the portion of the matrix metalingot within the inner boundaries of the GRAFOIL® frame.

The temperature within the resistance heated controlled atmospherefurnace was maintained at about 225° C. for about 2 hours. An about 7inch (178 mm) by about 61/2 inch (165 mm) by about 2 inch (51 mm) thicklayer of CERABLANKET® ceramic insulation material (Manville RefractoryProducts, Denver, Colo.) was prepared by cutting out a rectangularsection measuring about 5 inches (127 mm) by about 41/2 inches (114 mm)from the center of the ceramic insulation material to form a fiberblanket window frame. After the graphite boat and its contents wereremoved from the furnace and allowed to cool to about 550° C. (when thematrix metal solidified) the fiber blanket window frame was centered ontop of the now formed metal matrix composite and the assembly wasallowed to cool to room temperature. Upon reaching room temperature, thefiber blanket window frame was removed from the metal matrix compositebody and the metal matrix composite body was removed by hand from thesolidified matrix metal. FIG. 9 is a photograph of the surface of themetal matrix composite body that had been in contact with the gatingmeans which controlled the areal contact between the filler material andthe matrix metal.

Sample B

A tape cast silicon carbide preform having dimensions of about 8 inches(203 mm) by about 7 inches (177 mm) by about 0.08 inch (2 mm) thick andcomprising by weight about 70% 220 grit, 10% 500 grit, 10% 800 grit, and10% 1000 grit 39 CRYSTOLON® green silicon carbide particulate (NortonCompany, Worcester, Mass.) was placed, with its flatest side facingdown, on a perforated cordierite plate. The preform was covered with asheet of FIBERFRAX® 907-J fiber insulation paper (The CarborundumCompany, Niagara Falls, N.Y.). A second cordierite plate was placed ontop of the fiber insulation paper to form an assembly. The assembly wasplaced within a room temperature air atmosphere furnace. The temperaturein the furnace was increased from about room temperature to about 425°C. in about 8 hours. After maintaining a temperature of about 425° C.for about 1 hour, the temperature in the furnace was increased to about1050° C. in about 3 hours. After maintaining a temperature of about1050° C. for about 2 hours, the temperature in the furnace was decreasedto about room temperature in about 5 hours. The assembly was removedfrom the furnace, and the weight of the preform was determined to beabout 160.03 grams.

A mixture comprising by volume about 50% DAG® 154 colloidal graphite(Acheson Colloids, Port Huron, Mich.) and about 50% denatured ethanolwas prepared. An air brush was used to apply a thin layer of barriercoating to one 8 inch (203 mm) by 7 inch (177 mm) side of the preform.The coating was allowed to dry, and two additional coatings were appliedin this manner. The preform was placed on a balance and a preform weightof about 161.1 grams was recorded.

A sheet of GRAFOIL® graphite foil (Union Carbide Company, Danbury,Conn.) measuring about 131/4 inches (337 mm) by about 91/4 (235 mm)inches by about 0.015 inch (0.4 mm) thick was placed into the bottom ofa graphite boat having inner dimensions of about 131/4 inches (337 mm)by about 91/4 inches (235 mm) by about 1 inch (25 mm) high. An about3/8" (9 mm) thick layer of bedding material, comprising by weight about13% F-69 borosilicate glass frit (Fusion Ceramics, Inc., Carrollton,Ohio) and the balance 90 grit E-38 ALUNDUM® alumina (Norton Company,Worcester, Mass.) was poured into the graphite boat on top of theGRAFOIL® sheet. A foam brush was used to establish a level layer ofbedding material.

A matrix metal ingot weighing about 2600 grams and comprising by weightabout 20% silicon, 5% magnesium and the balance aluminum, was placedinto an ethanol bath. The surface of the matrix metal ingot was cleanedby hand utilizing a paper towel, then the matrix metal ingot was removedfrom the ethanol bath and placed within an air atmosphere furnace. Atemperature of about 68° C. was established within the furnace, andafter heating the matrix metal ingot for about 10 minutes, the matrixmetal ingot was removed from the furnace and placed on top of thebedding material within the graphite boat. Additional bedding materialwas poured into the graphite boat around the matrix metal ingot to alevel substantially the same as the top portion of the matrix metalingot. About 5.6 grams of -50 mesh atomized magnesium (Hart Corporation,Tamaqua, Pa.) was sprinkled onto the top portion of the matrix metalingot. The preform was placed into the graphite boat, with the uncoatedside of the preform in contact with the -50 mesh atomized magnesium andthe matrix metal ingot.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at about room temperature. The furnace wassealed, evacuated to about 30 inches (762 mm) of mercury vacuum, andbackfilled with nitrogen gas to about atmospheric pressure. A nitrogengas flow rate of about 5 liters per minute was established within thefurnace. The temperature in the furnace was increased from about roomtemperature to about 225° C. at a rate of about 200° C. per hour. Aftermaintaining a temperature of about 225° C. for about 52 hours, thetemperature in the furnace was increased to about 850° C. at a rate ofabout 200° C. per hour. After maintaining a temperature of about 850° C.for about 7 hours, the temperature in the furnace was decreased to about825° C. at a rate of about 200° C. per hour. The graphite boat and itscontents were then removed from the furnace. An about 15 inch (381 mm)by about 11 inch (279 mm) by about 2 inch (51 mm) thick layer ofCERABLANKET® ceramic insulation material (Manville Refractory Products,Denver, Colo.) was placed onto a graphite table. A single sheet ofGRAFOIL® graphite foil having dimensions of about 15 inches (381 mm) byabout 11 inches (279 mm) by about 0.015 inch (0.38 mm) thick was placedon top of the CERABLANKET® fiber insulation material. The graphite boatand its contents were placed on top of the GRAFOIL® graphite foil and amodel 500 air gun (Milwaukee Heat Tools, Inc., Milwaukee, Wis.) was setto cold and turned on. The cool air stream was directed at the formedmetal matrix composite and the matrix metal carcass to assist in coolingthe assembly. After about 3 minutes, the metal matrix composite wasremoved from the graphite boat, utilizing a stainless steel spatula,placed on the graphite table, and allowed to cool to room temperature.FIG. 10 is a photograph of the surface of the metal matrix compositethat had been in contact with the matrix metal.

EXAMPLE 10

About 166.5 grams of a mixture comprising by weight about 30% AIRVOL®PVA (Air Products and Chemicals, Inc., Allentown, Pa.) and about 70%deionized water was placed into a plastic jar. About 24.9 grams ofpolyethylene glycol 400 (J. T. Baker, Inc., Jackson, Tenn.), about 2.4grams of zinc stearate (Fischer Scientific, Pittsburgh, Pa.) and about106.2 grams of LUDOX® SM (E. I. DuPont DeNemours and Co., Inc.,Wilmington, Del.) were added to the jar. A hand-held drill with animpeller attachment was used to thoroughly mix the contents of the jarto prepare a binder solution.

About 1750 grams of 320 grit, about 250 grams of 800 grit, and about 250grams of 1000 grit 39 CRYSTOLON® green silicon carbide particulate(Norton Company, Worcester, Mass.), and about 250 grams of LC12N Si₃ N₄powder (Herman C. Stark, Berlin, Germany) were added to a one gallonplastic jar (Fischer Scientific, Pittsburgh, Pa.) which was then placedon a jar mill and roll mixed for about two hours. The plastic jar andits contents were then removed from the jar mill and the contents of thejar were poured into a Model RV02 Eirich mixer (Eirich Machines, Maple,Ontario, Canada). About 100 grams of the binder solution was poured intothe mixer, and the mixer was turned on with the pan and rotor settingsset to fast. After about one minute, the mixer was turned off, a plasticstraight edge was utilized to remove any silicon carbide particulate orsilicon nitride powder from the sides of the mixer bowl, and anadditional about 100 grams of binder solution was added to the mixer.The mixer was turned on a second time with the pan and rotor settingsset to fast. After about one minute, the mixer was turned off, a plasticstraight edge was used to scrape any silicon carbide particulate orsilicon nitride powder from the sides of the bowl, and an additionalabout 100 grams of binder solution was added to the mixer. The mixer wasturned on a third time with the pan and rotor settings set to fast.After about one minute, the mixer was turned off and the binder/siliconcarbide particulate/silicon nitride powder mixture was poured onto atable that had been previously covered with brown paper. An about 1/8inch (3 mm) to about 1/4 inch (6 mm) thick layer of the mixture wasestablished on the brown paper and the mixture was allowed to dryovernight.

The mixture was placed into a Model B Ro-tap testing sieve shaker (TylerCombustion Engineering, Inc.) and sifted through a 25 mesh screen. About160 grams of the mixture was placed into a die mold having dimensions ofabout 3 inches (76 mm) square and pressed at about 90 tons of pressureutilizing a CARVER® air type hydraulic press (Fred S. Carver, Inc.,Menomonee Falls, Wis.). The resultant preform was removed from thehydraulic press and preform dimensions of about 3 inches (76 mm) squareby about 1/2 inch (13 mm) thick were recorded. A total of four preformswere prepared in this manner. The preforms were then placed onto arefractory support plate, which had been covered with a sheet ofFIBERFRAX® 907-J fiber insulation paper (The Carborundum Company,Niagara Falls, N.Y.). The supported preforms were placed into a roomtemperature resistance heated air atmosphere furnace. The furnacetemperature was raised from about room temperature to about 500° C. at arate of about 100° C. per hour. After maintaining a temperature of about500° C. for about two hours, the temperature was then increased fromabout 500° C. to about 850° C. at a rate of about 200° C. per hour.After maintaining a temperature of about 850° C. for about four hours,the temperature was decreased to about room temperature in about fivehours.

A foam brush was utilized to apply a uniform coating of DAG® 154colloidal graphite (Acheson Colloids Co., Port Huron, Mich.) to the four1/2 inch (13 mm) thick sides and one 3 inch (76 mm) square side of eachpreform. The coating was allowed to dry and a second coating of DAG® 154colloidal graphite was applied to the five sides of each preformpreviously coated. Each preform was turned over and the final side wascoated with DAG® 154 colloidal graphite. However, before the DAG® 154colloidal graphite on the final side could substantially dry, excessDAG® 154 was removed from the surface of the preform utilizing a papertowel thereby forming a very thin coating. A second coating of DAG® 154colloidal graphite was applied to the final side of each preform and apaper towel was again used to remove excess coating before it dried,again forming a very thin coating for use as a gating means.

A graphite boat having internal dimensions of about 10 inches (254 mm)square by about 4 inches (102 mm) deep was prepared by lining the bottomof the graphite boat with a sheet of GRAFOIL® graphite foil (UnionCarbide Company, Danbury, Conn.) having dimensions of about 10 inches(254 mm) square by about 0.015 inch (0.38 mm) thick. An about 1 inch (25mm) thick layer of a bedding material comprising by weight about 971/2%90 grit E1 ALUNDUM® alumina (Norton Company, Worcester, Mass.) and about21/2% F-69 borosilicate glass frit (Fusion Ceramics, Inc., Carrollton,Ohio) was poured into the graphite boat and onto the GRAFOIL® sheet. Afoam brush was used to establish a level layer of bedding material. Amatrix metal ingot having dimensions of about 7 inches (178 mm) squareby about 1/2 inch (13 mm) high, weighing about 1003 grams and comprisingby weight about 15% silicon, 5% magnesium and the balance aluminum, wasplaced into the graphite boat and onto the bedding material. Fourequally spaced holes measuring about 27/8 inches (73 mm) square were cutout of a GRAFOIL® graphite foil sheet having dimensions of about 7inches (178 mm) square by about 0.015 inch (0.38 mm) thick. The GRAFOIL®sheet was placed into the graphite boat and centered over the matrixmetal ingot. A total of about 1.8 grams of -50 mesh atomized magnesium(Hart Corporation, Tamaqua, Pa.) was evenly dispersed throughout thefour holes in the GRAFOIL® sheet. The four preforms were placed into thegraphite boat so that one preform was centered over each hole in theGRAFOIL® sheet and such that the final side (as identified above) ofeach preform containing the gating means was in contact with themagnesium layer.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at about room temperature. The furnace wassealed, evacuated to about 30 inches (762 mm) of mercury vacuum, andbackfilled with nitrogen gas to about atmospheric pressure. A nitrogengas flow rate of about 5 liters per minute was established within thefurnace. The temperature in the furnace was increased from about roomtemperature to about 200° C. at a rate of about 200° C. per hour. Aftermaintaining a temperature of about 200° C. for about six hours, thetemperature in the furnace was increased to about 550° C. at a rate ofabout 200° C. per hour. After maintaining a temperature of about 550° C.for about two hours, the temperature in the furnace was increased toabout 875° C. at a rate of about 200° C. per hour. After maintaining atemperature of about 875° C. for about 30 hours, the temperature in thefurnace was decreased to about 700° C. at a rate of about 200° C. perhour. The graphite boat and its contents were then removed from thefurnace. An about 10 inch (254 mm) square sheet of FIBERFRAX® 907-Jfiber insulation paper (Carborundum Company, Niagara Falls, N.Y.) wasplaced onto a graphite table. The graphite boat and its contents wereplaced on top of the FIBERFRAX® insulation material and allowed to coolto room temperature. Upon reaching room temperature, the formed metalmatrix composite bodies were easily removed from the solidified matrixmetal by inverting the solidified matrix metal and applying light hammerblows to the bottom of the matrix metal, yielding a metal matrixcomposite with a net or near-net surface finish on the side thatcontacted the gating means.

EXAMPLE 11

An aqueous solution of BLUONIC® A colloidal alumina (Wesbond Corp.,Wilmington, Del.) weighing about 1034 grams was diluted with about 2082grams of deionized water and placed into a 5 liter NALGENE® plastic jar(Nalge Company, Rochester, N.Y.). About 7267 grams of 220 grit and about3116 grams of 500 grit 39 CRYSTOLON® green silicon carbide particulate(Norton Company, Worcester, Mass.), and about 109 grams of colloids581-B defoamer (Colloids, Inc., Newark, N.J.) were added to the jar toprepare a slurry for sediment casting. The jar and its contents wereplaced on a jar mill and roll mixed for about 5 hours.

A Grade GI-1000 silicone rubber mold (Plastic Tooling Supply Company,Exton, Pa.) having a hexagonal shaped internal cavity with sidesmeasuring about 21/4 inches (57 mm) and a depth of about 2 inches (51mm) was placed onto a flat rigid aluminum plate. The mold/plate assemblywas then placed onto a level vibrating table. The vibrating table wasturned on and approximately 680 grams of the slurry were poured into themold in a smooth and continuous manner. The mold and its contents weresubjected to vibration for at least about 1 hour to condense the slurryinto a preform, with excess surface liquid being removed with a sponge.The vibrating table was turned off and the mold/plate/preform assemblywas placed into a freezer. Residual water in the preform was permittedto freeze thoroughly, then the mold/plate/preform assembly was removedfrom the freezer and the frozen sediment cast preform, with sidesmeasuring about 21/4 inches (57 mm) and a thickness of about 3/4 inch(19 mm), was removed from the mold. The preform was then placed onto arefractory support plate, which had been covered with a sheet ofFIBERFRAX® 907-J fiber insulation paper (The Carborundum Company,Niagara Falls, N.Y.). A temperature of about 25° C. was establishedwithin a resistance heated air atmosphere furnace, and the supportedpreform was placed into the furnace. After about 12 hours, the furnacetemperature was increased from about 25° C. to about 85° C. in about 2hours. After maintaining a temperature of about 85° C. for about 12hours, the temperature was then increased from about 85° C. to about1050° C. in about 10 hours. After maintaining a temperature of about1050° C. for about 2 hours, the temperature was decreased to about roomtemperature in about 10 hours.

The refractory plate/fiber insulation paper/preform assembly was removedfrom the furnace, and the top portion of the preform was lightly sandedto establish a flat surface. A mixture to be used as a barrier coatingcomprising by volume about 50% DAG® 154 colloidal graphite (AchesonColloids, Port Huron, Mich.) and about 50% denatured ethanol wasprepared. The preform was placed on a table with the sanded side facingdown. A foam brush was used to apply a thin layer of barrier coating tothe exposed surfaces of the preform. The barrier coating was allowed tosubstantially dry, then the procedure was repeated a second time. Thepreform was turned over and a thin coating of a gating means mixture,comprising by weight about 33.7% A-1000 alumina (Aluminum Company ofAmerica, Pittsburgh Pa.), about 67% denatured ethanol, and about 0.3%HYPERMER® KD2 dispersant (ICI Americas, Wilmington, Del.), was appliedto the sanded surface of the preform utilizing a foam brush. The gatingmeans mixture was allowed to dry. A total of 3 coatings of the gatingmeans mixture were applied in this manner.

A graphite foil box measuring about 111/4 inches (286 mm) by about 61/4inches (159 mm) and about 7 inches (178 mm) high, was fabricated from asingle sheet of GRAFOIL® graphite foil (Union Carbide Company, Danbury,Conn.) measuring about 0.015 inch (0.38 mm) thick by makingstrategically located cuts and folds in the sheet. The folds in theGRAFOIL® sheet were cemented together with RIGIDLOCK® graphite cement(Polycarbon Corp., Valencia, Calif.). Strategically placed stapleshelped to reinforce the graphite cement. The GRAFOIL® box was thenplaced within a graphite boat having inner dimensions which weresubstantially the same as the outer dimensions of the GRAFOIL® box.

An about 1 inch (25 mm) thick layer of bedding material, comprising byweight about 9% F69 borosilicate glass frit (Fusion Ceramics, Inc.,Carrollton, Ohio), about 27% 90 grit E1 ALUNDUM® alumina (NortonCompany, Worcester, Mass.), and about 64% 36 grit E38 ALUNDUM® alumina(Norton Company) was poured into the GRAFOIL® box and leveled. A matrixmetal ingot having dimensions of about 31/2 inches (89 mm) by about 4inches (102 mm) by about 2 inches (51 mm) and weighed about 1250 gramsand comprising by weight about 121/2% silicon, 5% magnesium and thebalance aluminum, was placed into the GRAFOIL® box and onto the beddingmaterial. A hexagonal shaped commercially available 6061 aluminum sheetwith sides measuring about 21/4 inches (57 mm) and about 0.05 inch (1mm) thick, was placed into the GRAFOIL® box and centered over the matrixmetal ingot. Additional bedding material was poured into the GRAFOIL®box around the matrix metal ingot to a level substantially flush withthe top of the aluminum sheet. About 2 grams of -50 mesh groundmagnesium (Hart Corp., Tamaqua, Pa.) was evenly dispersed onto thealuminum sheet. The preform was placed into the GRAFOIL® box with thesanded side in contact with the magnesium.

The graphite boat and its contents were placed into a resistance heatedcontrolled atmosphere furnace at about room temperature. The furnace wassealed, evacuated to about 30 inches (762 mm) of mercury vacuum andheated to about 50° C. in about 1 hour. After maintaining a temperatureof about 50° C. and a vacuum level of about 30 inches (762 mm) ofmercury vacuum for about 1 hour, the furnace was backfilled withnitrogen gas to about atmospheric pressure and a nitrogen gas flow rateof about 250 cubic feet per hour (2 liters per second) was established.The temperature within the furnace was increased to about 100° C. inabout 1/2 hour. After maintaining a temperature of about 100° C. forabout 1 hour, the temperature was increased to about 200° C. in aboutone hour. After maintaining a temperature of about 200° C. for about 1hour, the temperature in the furnace was increased to about 800° C. inabout 3 hours. After maintaining a temperature of about 800° C. forabout 12 hours, the temperature in the furnace was decreased to aboutroom temperature in about 4 hours. The furnace door was opened, and thegraphite boat and its contents were removed from the furnace and placedon a table. The formed metal matrix composite was separated from thesolidified matrix metal carcass by applying light hammer blows to thesolidified matrix metal carcass.

We claim:
 1. An article, comprising:a metal body; a metal matrixcomposite body comprising at least one filler material dispersed in amatrix metal; a gating means comprising graphite and featuring aplurality of microchannels therethrough, said gating means beingdisposed between said metal body and said metal matrix composite body,wherein said gating means reduces a strength of a bond between saidmetal body and said metal matrix composite body; and a plurality ofcontinuous metal pathways from said metal body to said metal matrixcomposite body through at least a portion of said plurality ofmicrochannels.
 2. The article of claim 1, wherein said graphitecomprises colloidal-size particles.
 3. The article of claim 1, whereinsaid gating means is present in coating form.
 4. The article of claim 1,wherein said graphite is disposed on at least one surface of said metalmatrix composite body.
 5. The article of claim 1, wherein said metalbody comprises aluminum and at least one auxiliary alloying elementselected from the group consisting of iron, silicon, copper, magnesium,manganese, chromium, zinc, calcium and strontium.
 6. The article ofclaim 1, wherein said at least one filler material is provided in a formselected from the group consisting of powders, flakes, whiskers, pelletsand refractory cloths.
 7. The article of claim 1, wherein said at leastone filler material is provided in a form selected from the groupconsisting of platelets, fibers, particulates and spheres.
 8. Anarticle, comprising:a carcass of matrix metal; a metal matrix compositebody comprising at least one filler material distributed in a matrixmetal; an at least partially permeable gating means comprising graphitedisposed between said metal matrix composite body and said carcass ofmatrix metal, whereby said gating means facilitates separation of saidmetal matrix composite body from said carcass of matrix metal.
 9. Thearticle of claim 8, wherein said graphite comprises colloidal graphite,and further wherein said gating means reduces a strength of a bondbetween said metal matrix composite body and said carcass of matrixmetal.
 10. The article of claim 8, further comprising at least onecontinuous pathway comprising matrix metal extending from said carcassof matrix metal through said gating means to said metal matrix compositebody.